diff --git a/UPGRADE_2_TO_3.md b/UPGRADE_2_TO_3.md
new file mode 100644
index 0000000..0f60442
--- /dev/null
+++ b/UPGRADE_2_TO_3.md
@@ -0,0 +1,31 @@
+# Upgrade from library v2 to v3
+
+## Enums
+
+Content types:
+- `Document::CONTENT_TYPE_OFFICIAL` to `ContentTypeEnum::Official`
+- `Document::CONTENT_TYPE_DEBUG` to `ContentTypeEnum::Debug`
+- `Document::CONTENT_TYPE_JSONP` to `ContentTypeEnum::Jsonp`
+
+Jsonapi versions:
+- `Document::JSONAPI_VERSION_1_0` to `JsonapiVersionEnum::V_1_0`
+- `Document::JSONAPI_VERSION_1_1` to `JsonapiVersionEnum::V_1_1`
+- `Document::JSONAPI_VERSION_LATEST` to `JsonapiVersionEnum::Latest`
+
+Document levels:
+- `Document::LEVEL_ROOT` to `DocumentLevelEnum::Root`
+- `Document::LEVEL_JSONAPI` to `DocumentLevelEnum::Jsonapi`
+- `Document::LEVEL_Resource` to `DocumentLevelEnum::Resource`
+
+Sorting orders:
+- `RequestParser->getSortFields()` returns a `SortOrderEnum` instead of `string` for the `order` field
+
+Relationship types:
+- `RelationshipObject::TO_ONE` to `RelationshipTypeEnum::ToOne`
+- `RelationshipObject::TO_MANY` to `RelationshipTypeEnum::ToMany`
+
+## Internal
+
+Enums:
+- `RequestParser::SORT_*` to `SortOrderEnum::*`
+- `Validator::OBJECT_CONTAINER_*` to `ObjectContainerEnum::*`
diff --git a/examples/atomic_operations_extension.php b/examples/atomic_operations_extension.php
index 4e97c6d..caa6aaa 100644
--- a/examples/atomic_operations_extension.php
+++ b/examples/atomic_operations_extension.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\objects\ResourceObject;
 use alsvanzelf\jsonapi\extensions\AtomicOperationsDocument;
 
diff --git a/examples/bootstrap_examples.php b/examples/bootstrap_examples.php
index 98d2a93..a073357 100644
--- a/examples/bootstrap_examples.php
+++ b/examples/bootstrap_examples.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\Document;
 use alsvanzelf\jsonapi\ResourceDocument;
 use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
diff --git a/examples/collection.php b/examples/collection.php
index cf2f47a..6f71483 100644
--- a/examples/collection.php
+++ b/examples/collection.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\objects\ResourceObject;
 
diff --git a/examples/collection_canonical.php b/examples/collection_canonical.php
index 92fc821..b9fad86 100644
--- a/examples/collection_canonical.php
+++ b/examples/collection_canonical.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\objects\ResourceObject;
 
diff --git a/examples/cursor_pagination_profile.php b/examples/cursor_pagination_profile.php
index c70fef7..065a4d3 100644
--- a/examples/cursor_pagination_profile.php
+++ b/examples/cursor_pagination_profile.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\objects\ResourceObject;
 use alsvanzelf\jsonapi\profiles\CursorPaginationProfile;
diff --git a/examples/errors_all_options.php b/examples/errors_all_options.php
index e43bf4f..3e71b2f 100644
--- a/examples/errors_all_options.php
+++ b/examples/errors_all_options.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ErrorsDocument;
 use alsvanzelf\jsonapi\objects\ErrorObject;
 
diff --git a/examples/errors_exception_native.php b/examples/errors_exception_native.php
index 59fde02..9f2b405 100644
--- a/examples/errors_exception_native.php
+++ b/examples/errors_exception_native.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ErrorsDocument;
 
 require 'bootstrap_examples.php';
diff --git a/examples/extension.php b/examples/extension.php
index 74738bf..42f87e6 100644
--- a/examples/extension.php
+++ b/examples/extension.php
@@ -1,7 +1,9 @@
 <?php
 
-use alsvanzelf\jsonapi\Document;
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
 use alsvanzelf\jsonapi\helpers\Converter;
 
 require 'bootstrap_examples.php';
@@ -28,7 +30,7 @@
  * get the json
  */
 
-$contentType = Converter::prepareContentType(Document::CONTENT_TYPE_OFFICIAL, [$extension], []);
+$contentType = Converter::prepareContentType(ContentTypeEnum::Official, [$extension], []);
 echo '<code>Content-Type: '.$contentType.'</code>'.PHP_EOL;
 
 $options = [
diff --git a/examples/meta_only.php b/examples/meta_only.php
index a41df02..60562d7 100644
--- a/examples/meta_only.php
+++ b/examples/meta_only.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\MetaDocument;
 
 require 'bootstrap_examples.php';
diff --git a/examples/null_values.php b/examples/null_values.php
index 537195d..4a7cd92 100644
--- a/examples/null_values.php
+++ b/examples/null_values.php
@@ -1,6 +1,9 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\LinkObject;
 use alsvanzelf\jsonapi\objects\RelationshipObject;
 
@@ -22,8 +25,8 @@
 
 // show a relationship is not set
 $document->addRelationship('bar', null);
-$document->addRelationshipObject('baz', new RelationshipObject(RelationshipObject::TO_ONE));
-$document->addRelationshipObject('baf', new RelationshipObject(RelationshipObject::TO_MANY));
+$document->addRelationshipObject('baz', new RelationshipObject(RelationshipTypeEnum::ToOne));
+$document->addRelationshipObject('baf', new RelationshipObject(RelationshipTypeEnum::ToMany));
 
 /**
  * sending the response
diff --git a/examples/output.php b/examples/output.php
index 0844c33..f6726e8 100644
--- a/examples/output.php
+++ b/examples/output.php
@@ -1,7 +1,9 @@
 <?php
 
-use alsvanzelf\jsonapi\Document;
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\MetaDocument;
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
 
 require 'bootstrap_examples.php';
 
@@ -73,4 +75,4 @@
 echo '<pre>';
 $document->sendResponse($options);
 echo '</pre>';
-echo '<p><em>Also sends http status code ('.$document->getHttpStatusCode().') and headers: [Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.']</em></p>';
+echo '<p><em>Also sends http status code ('.$document->getHttpStatusCode().') and headers: [Content-Type: '.ContentTypeEnum::Official->value.']</em></p>';
diff --git a/examples/profile.php b/examples/profile.php
index 8b070b6..8245855 100644
--- a/examples/profile.php
+++ b/examples/profile.php
@@ -1,7 +1,9 @@
 <?php
 
-use alsvanzelf\jsonapi\Document;
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
 use alsvanzelf\jsonapi\helpers\Converter;
 
 require 'bootstrap_examples.php';
@@ -30,7 +32,7 @@
  * get the json
  */
 
-$contentType = Converter::prepareContentType(Document::CONTENT_TYPE_OFFICIAL, [], [$profile]);
+$contentType = Converter::prepareContentType(ContentTypeEnum::Official, [], [$profile]);
 echo '<code>Content-Type: '.$contentType.'</code>'.PHP_EOL;
 
 $options = [
diff --git a/examples/relationship_to_many_document.php b/examples/relationship_to_many_document.php
index 1f02263..f2a2af2 100644
--- a/examples/relationship_to_many_document.php
+++ b/examples/relationship_to_many_document.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\CollectionDocument;
 
 require 'bootstrap_examples.php';
diff --git a/examples/relationship_to_one_document.php b/examples/relationship_to_one_document.php
index 907e0b5..7dfc6eb 100644
--- a/examples/relationship_to_one_document.php
+++ b/examples/relationship_to_one_document.php
@@ -1,7 +1,9 @@
 <?php
 
-use alsvanzelf\jsonapi\Document;
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 
 require 'bootstrap_examples.php';
 
@@ -11,7 +13,7 @@
 
 $relationshipDocument = new ResourceDocument('author', 12);
 
-$relationshipDocument->setSelfLink('/articles/1/relationship/author', $meta=[], $level=Document::LEVEL_ROOT);
+$relationshipDocument->setSelfLink('/articles/1/relationship/author', $meta=[], $level=DocumentLevelEnum::Root);
 $relationshipDocument->addLink('related', '/articles/1/author');
 
 /**
diff --git a/examples/relationships.php b/examples/relationships.php
index bc25dfe..6755f34 100644
--- a/examples/relationships.php
+++ b/examples/relationships.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\RelationshipObject;
 use alsvanzelf\jsonapi\objects\ResourceObject;
 
@@ -45,7 +48,7 @@
  * to-many relationship, one-by-one
  */
 
-$relationshipObject = new RelationshipObject($type=RelationshipObject::TO_MANY);
+$relationshipObject = new RelationshipObject($type=RelationshipTypeEnum::ToMany);
 $relationshipObject->addResource($friend1Resource);
 $relationshipObject->addResource($friend2Resource);
 
@@ -65,7 +68,7 @@
  * to-many relationship, different types
  */
 
-$relationshipObject = new RelationshipObject($type=RelationshipObject::TO_MANY);
+$relationshipObject = new RelationshipObject($type=RelationshipTypeEnum::ToMany);
 $relationshipObject->addResource($ship1Resource);
 $relationshipObject->addResource($dockResource);
 
diff --git a/examples/request_superglobals.php b/examples/request_superglobals.php
index fff5992..cb93d06 100644
--- a/examples/request_superglobals.php
+++ b/examples/request_superglobals.php
@@ -1,6 +1,8 @@
 <?php
 
-use alsvanzelf\jsonapi\Document;
+declare(strict_types=1);
+
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
 use alsvanzelf\jsonapi\helpers\RequestParser;
 
 require 'bootstrap_examples.php';
@@ -44,7 +46,7 @@
 $_SERVER['REQUEST_SCHEME'] = 'https';
 $_SERVER['HTTP_HOST']      = 'example.org';
 $_SERVER['REQUEST_URI']    = '/user/42?'.http_build_query($_GET);
-$_SERVER['CONTENT_TYPE']   = Document::CONTENT_TYPE_OFFICIAL;
+$_SERVER['CONTENT_TYPE']   = ContentTypeEnum::Official->value;
 
 /**
  * parsing the request
diff --git a/examples/resource_human_api.php b/examples/resource_human_api.php
index 67ed657..b9ae514 100644
--- a/examples/resource_human_api.php
+++ b/examples/resource_human_api.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
 use alsvanzelf\jsonapi\objects\AttributesObject;
 use alsvanzelf\jsonapi\objects\LinksObject;
diff --git a/examples/resource_links.php b/examples/resource_links.php
index adedd1a..a79e1f8 100644
--- a/examples/resource_links.php
+++ b/examples/resource_links.php
@@ -1,7 +1,9 @@
 <?php
 
-use alsvanzelf\jsonapi\Document;
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 
 require 'bootstrap_examples.php';
 
@@ -13,13 +15,13 @@
  */
 $document = ResourceDocument::fromObject($userEntity, $type='user', $userEntity->id);
 
-$selfResourceMeta = ['level' => Document::LEVEL_RESOURCE];
-$partnerMeta      = ['level' => Document::LEVEL_RESOURCE];
-$redirectMeta     = ['level' => Document::LEVEL_ROOT];
+$selfResourceMeta = ['level' => DocumentLevelEnum::Resource->name];
+$partnerMeta      = ['level' => DocumentLevelEnum::Resource->name];
+$redirectMeta     = ['level' => DocumentLevelEnum::Root->name];
 
 $document->setSelfLink('/user/42',        $selfResourceMeta);
-$document->addLink('partner',  '/user/1', $partnerMeta,  $level=Document::LEVEL_RESOURCE);
-$document->addLink('redirect', '/login',  $redirectMeta, $level=Document::LEVEL_ROOT);
+$document->addLink('partner',  '/user/1', $partnerMeta,  $level=DocumentLevelEnum::Resource);
+$document->addLink('redirect', '/login',  $redirectMeta, $level=DocumentLevelEnum::Root);
 
 /**
  * sending the response
diff --git a/examples/resource_nested_relations.php b/examples/resource_nested_relations.php
index b0418f3..e182a48 100644
--- a/examples/resource_nested_relations.php
+++ b/examples/resource_nested_relations.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
 use alsvanzelf\jsonapi\objects\ResourceObject;
 
diff --git a/examples/resource_spec_api.php b/examples/resource_spec_api.php
index b826082..fa160d4 100644
--- a/examples/resource_spec_api.php
+++ b/examples/resource_spec_api.php
@@ -1,6 +1,9 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\AttributesObject;
 use alsvanzelf\jsonapi\objects\LinksObject;
 use alsvanzelf\jsonapi\objects\MetaObject;
@@ -41,7 +44,7 @@
 $resource->setType('user');
 $resource->setAttributesObject($attributes42);
 
-$relationship = new RelationshipObject(RelationshipObject::TO_ONE);
+$relationship = new RelationshipObject(RelationshipTypeEnum::ToOne);
 $relationship->setResource($resource);
 $relationships = new RelationshipsObject();
 $relationships->addRelationshipObject('friend', $relationship);
diff --git a/examples/status_only.php b/examples/status_only.php
index abcced8..94439c0 100644
--- a/examples/status_only.php
+++ b/examples/status_only.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use alsvanzelf\jsonapi\MetaDocument;
 
 require 'bootstrap_examples.php';
diff --git a/rector.php b/rector.php
index 9556204..97960dd 100644
--- a/rector.php
+++ b/rector.php
@@ -3,6 +3,7 @@
 declare(strict_types=1);
 
 use Rector\Config\RectorConfig;
+use Rector\TypeDeclaration\Rector\StmtsAwareInterface\DeclareStrictTypesRector;
 
 // @see https://github.com/rectorphp/rector/blob/main/docs/rector_rules_overview.md for more rules
 
@@ -12,7 +13,10 @@
 		__DIR__ . '/tests',
 		__DIR__ . '/examples',
 	])
-
+	->withRules([
+		DeclareStrictTypesRector::class,
+	])
+	
 	// tab-based indenting
 	->withIndent(indentChar: "\t", indentSize: 1)
 
diff --git a/src/CollectionDocument.php b/src/CollectionDocument.php
index ba7c458..55cae30 100644
--- a/src/CollectionDocument.php
+++ b/src/CollectionDocument.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi;
 
 use alsvanzelf\jsonapi\DataDocument;
diff --git a/src/DataDocument.php b/src/DataDocument.php
index 470fcef..6d10d2e 100644
--- a/src/DataDocument.php
+++ b/src/DataDocument.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi;
 
 use alsvanzelf\jsonapi\Document;
diff --git a/src/Document.php b/src/Document.php
index cd26fcc..8c1bea3 100644
--- a/src/Document.php
+++ b/src/Document.php
@@ -1,7 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi;
 
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\InputException;
@@ -30,18 +34,6 @@ abstract class Document implements DocumentInterface, \JsonSerializable, HasLink
 		LinksManager::addLink as linkManagerAddLink;
 	}
 	
-	const JSONAPI_VERSION_1_0 = '1.0';
-	const JSONAPI_VERSION_1_1 = '1.1';
-	const JSONAPI_VERSION_LATEST = Document::JSONAPI_VERSION_1_1;
-	
-	const CONTENT_TYPE_OFFICIAL = 'application/vnd.api+json';
-	const CONTENT_TYPE_DEBUG    = 'application/json';
-	const CONTENT_TYPE_JSONP    = 'application/javascript';
-	
-	const LEVEL_ROOT     = 'root';
-	const LEVEL_JSONAPI  = 'jsonapi';
-	const LEVEL_RESOURCE = 'resource';
-	
 	/** @var MetaObject */
 	protected $meta;
 	/** @var ?JsonapiObject */
@@ -66,7 +58,7 @@ abstract class Document implements DocumentInterface, \JsonSerializable, HasLink
 		 * send out the official jsonapi content-type header
 		 * overwrite for jsonp or if clients don't support it
 		 */
-		'contentType' => Document::CONTENT_TYPE_OFFICIAL,
+		'contentType' => ContentTypeEnum::Official,
 		
 		/**
 		 * overwrite the array to encode to json
@@ -97,23 +89,15 @@ public function __construct() {
 	 * @param string $key
 	 * @param string $href
 	 * @param array  $meta optional, if given a LinkObject is added, otherwise a link string is added
-	 * @param string $level one of the Document::LEVEL_* constants, optional, defaults to Document::LEVEL_ROOT
 	 * 
-	 * @throws InputException if the $level is Document::LEVEL_JSONAPI, Document::LEVEL_RESOURCE, or unknown
+	 * @throws InputException if the $level is not DocumentLevelEnum::Root
 	 */
-	public function addLink($key, $href, array $meta=[], $level=Document::LEVEL_ROOT) {
-		if ($level === Document::LEVEL_ROOT) {
-			$this->linkManagerAddLink($key, $href, $meta);
-		}
-		elseif ($level === Document::LEVEL_JSONAPI) {
-			throw new InputException('level "jsonapi" can not be used for links');
-		}
-		elseif ($level === Document::LEVEL_RESOURCE) {
-			throw new InputException('level "resource" can only be set on a ResourceDocument');
-		}
-		else {
-			throw new InputException('unknown level "'.$level.'"');
-		}
+	public function addLink($key, $href, array $meta=[], DocumentLevelEnum $level=DocumentLevelEnum::Root) {
+		match ($level) {
+			DocumentLevelEnum::Root     => $this->linkManagerAddLink($key, $href, $meta),
+			DocumentLevelEnum::Jsonapi  => throw new InputException('level "jsonapi" can not be used for links'),
+			DocumentLevelEnum::Resource => throw new InputException('level "resource" can only be set on a ResourceDocument'),
+		};
 	}
 	
 	/**
@@ -123,11 +107,10 @@ public function addLink($key, $href, array $meta=[], $level=Document::LEVEL_ROOT
 	 * 
 	 * @param string $href
 	 * @param array  $meta optional, if given a LinkObject is added, otherwise a link string is added
-	 * @param string $level one of the Document::LEVEL_* constants, optional, defaults to Document::LEVEL_ROOT
 	 */
-	public function setSelfLink($href, array $meta=[], $level=Document::LEVEL_ROOT) {
-		if ($level === Document::LEVEL_ROOT && ($this->extensions !== [] || $this->profiles !== [])) {
-			$contentType = Converter::prepareContentType(Document::CONTENT_TYPE_OFFICIAL, $this->extensions, $this->profiles);
+	public function setSelfLink($href, array $meta=[], DocumentLevelEnum $level=DocumentLevelEnum::Root) {
+		if ($level === DocumentLevelEnum::Root && ($this->extensions !== [] || $this->profiles !== [])) {
+			$contentType = Converter::prepareContentType(ContentTypeEnum::Official, $this->extensions, $this->profiles);
 			
 			$linkObject = new LinkObject($href, $meta);
 			$linkObject->setMediaType($contentType);
@@ -144,43 +127,42 @@ public function setSelfLink($href, array $meta=[], $level=Document::LEVEL_ROOT)
 	 * 
 	 * for example this could link to an OpenAPI or JSON Schema document
 	 * 
-	 * @note according to the spec, this can only be set to Document::LEVEL_ROOT
+	 * @note according to the spec, this can only be set to DocumentLevelEnum::Root
 	 * 
 	 * @param string $href
 	 * @param array  $meta optional, if given a LinkObject is added, otherwise a link string is added
 	 */
 	public function setDescribedByLink($href, array $meta=[]) {
-		$this->addLink('describedby', $href, $meta, $level=Document::LEVEL_ROOT);
+		$this->addLink('describedby', $href, $meta, $level=DocumentLevelEnum::Root);
 	}
 	
 	/**
 	 * @param string $key
 	 * @param mixed  $value
-	 * @param string $level one of the Document::LEVEL_* constants, optional, defaults to Document::LEVEL_ROOT
 	 * 
 	 * @throws InputException if the $level is unknown
-	 * @throws InputException if the $level is Document::LEVEL_RESOURCE
+	 * @throws InputException if the $level is DocumentLevelEnum::Resource
 	 */
-	public function addMeta($key, $value, $level=Document::LEVEL_ROOT) {
-		if ($level === Document::LEVEL_ROOT) {
+	public function addMeta($key, $value, DocumentLevelEnum $level=DocumentLevelEnum::Root) {
+		if ($level === DocumentLevelEnum::Root) {
 			if ($this->meta === null) {
 				$this->setMetaObject(new MetaObject());
 			}
 			
 			$this->meta->add($key, $value);
 		}
-		elseif ($level === Document::LEVEL_JSONAPI) {
+		elseif ($level === DocumentLevelEnum::Jsonapi) {
 			if ($this->jsonapi === null) {
 				$this->setJsonapiObject(new JsonapiObject());
 			}
 			
 			$this->jsonapi->addMeta($key, $value);
 		}
-		elseif ($level === Document::LEVEL_RESOURCE) {
+		elseif ($level === DocumentLevelEnum::Resource) {
 			throw new InputException('level "resource" can only be set on a ResourceDocument');
 		}
 		else {
-			throw new InputException('unknown level "'.$level.'"');
+			throw new InputException('unknown level "'.$level->value.'"');
 		}
 	}
 	
diff --git a/src/ErrorsDocument.php b/src/ErrorsDocument.php
index 69163db..48ef29e 100644
--- a/src/ErrorsDocument.php
+++ b/src/ErrorsDocument.php
@@ -1,9 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi;
 
 use alsvanzelf\jsonapi\Document;
-use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\objects\ErrorObject;
 
 /**
diff --git a/src/MetaDocument.php b/src/MetaDocument.php
index 299ad20..54f9946 100644
--- a/src/MetaDocument.php
+++ b/src/MetaDocument.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi;
 
 use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 use alsvanzelf\jsonapi\helpers\Converter;
 use alsvanzelf\jsonapi\objects\MetaObject;
 
@@ -41,9 +44,8 @@ public static function fromObject($meta) {
 	 * 
 	 * @param string $key
 	 * @param mixed  $value
-	 * @param string $level one of the Document::LEVEL_* constants, optional, defaults to Document::LEVEL_ROOT
 	 */
-	public function add($key, $value, $level=Document::LEVEL_ROOT) {
+	public function add($key, $value, DocumentLevelEnum $level=DocumentLevelEnum::Root) {
 		parent::addMeta($key, $value, $level);
 	}
 	
diff --git a/src/ResourceDocument.php b/src/ResourceDocument.php
index ff7ebca..085ef1a 100644
--- a/src/ResourceDocument.php
+++ b/src/ResourceDocument.php
@@ -1,10 +1,13 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi;
 
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\DataDocument;
 use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\helpers\Converter;
@@ -122,14 +125,13 @@ public function addRelationship($key, $relation, array $links=[], array $meta=[]
 	 * @param string $key
 	 * @param string $href
 	 * @param array  $meta optional, if given a LinkObject is added, otherwise a link string is added
-	 * @param string $level one of the Document::LEVEL_* constants, optional, defaults to Document::LEVEL_ROOT
 	 */
-	public function addLink($key, $href, array $meta=[], $level=Document::LEVEL_ROOT) {
+	public function addLink($key, $href, array $meta=[], DocumentLevelEnum $level=DocumentLevelEnum::Root) {
 		if ($this->resource instanceof ResourceObject === false) {
 			throw new Exception('the resource is an identifier-only object');
 		}
 		
-		if ($level === Document::LEVEL_RESOURCE) {
+		if ($level === DocumentLevelEnum::Resource) {
 			$this->resource->addLink($key, $href, $meta);
 		}
 		else {
@@ -143,12 +145,12 @@ public function addLink($key, $href, array $meta=[], $level=Document::LEVEL_ROOT
 	 * @param string $href
 	 * @param array  $meta optional
 	 */
-	public function setSelfLink($href, array $meta=[], $level=Document::LEVEL_RESOURCE) {
+	public function setSelfLink($href, array $meta=[], DocumentLevelEnum $level=DocumentLevelEnum::Resource) {
 		if ($this->resource instanceof ResourceObject === false) {
 			throw new Exception('the resource is an identifier-only object');
 		}
 		
-		if ($level === Document::LEVEL_RESOURCE) {
+		if ($level === DocumentLevelEnum::Resource) {
 			$this->resource->setSelfLink($href, $meta);
 		}
 		else {
@@ -159,10 +161,9 @@ public function setSelfLink($href, array $meta=[], $level=Document::LEVEL_RESOUR
 	/**
 	 * @param string $key
 	 * @param mixed  $value
-	 * @param string $level one of the Document::LEVEL_* constants, optional, defaults to Document::LEVEL_ROOT
 	 */
-	public function addMeta($key, $value, $level=Document::LEVEL_ROOT) {
-		if ($level === Document::LEVEL_RESOURCE) {
+	public function addMeta($key, $value, DocumentLevelEnum $level=DocumentLevelEnum::Root) {
+		if ($level === DocumentLevelEnum::Resource) {
 			$this->resource->addMeta($key, $value);
 		}
 		else {
diff --git a/src/enums/ContentTypeEnum.php b/src/enums/ContentTypeEnum.php
new file mode 100644
index 0000000..a3d23d1
--- /dev/null
+++ b/src/enums/ContentTypeEnum.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace alsvanzelf\jsonapi\enums;
+
+enum ContentTypeEnum: string {
+	case Official = 'application/vnd.api+json';
+	case Debug    = 'application/json';
+	case Jsonp    = 'application/javascript';
+}
diff --git a/src/enums/DocumentLevelEnum.php b/src/enums/DocumentLevelEnum.php
new file mode 100644
index 0000000..4fe102d
--- /dev/null
+++ b/src/enums/DocumentLevelEnum.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace alsvanzelf\jsonapi\enums;
+
+enum DocumentLevelEnum {
+	case Root;
+	case Jsonapi;
+	case Resource;
+}
diff --git a/src/enums/JsonapiVersionEnum.php b/src/enums/JsonapiVersionEnum.php
new file mode 100644
index 0000000..1a7186a
--- /dev/null
+++ b/src/enums/JsonapiVersionEnum.php
@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace alsvanzelf\jsonapi\enums;
+
+enum JsonapiVersionEnum: string {
+	case V_1_0  = '1.0';
+	case V_1_1  = '1.1';
+	case Latest = self::V_1_1->value;
+}
diff --git a/src/enums/ObjectContainerEnum.php b/src/enums/ObjectContainerEnum.php
new file mode 100644
index 0000000..d2258c0
--- /dev/null
+++ b/src/enums/ObjectContainerEnum.php
@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+namespace alsvanzelf\jsonapi\enums;
+
+/**
+ * @internal
+ */
+enum ObjectContainerEnum: string {
+	case Type          = 'type';
+	case Id            = 'id';
+	case Lid           = 'lid';
+	case Attributes    = 'attributes';
+	case Relationships = 'relationships';
+}
diff --git a/src/enums/RelationshipTypeEnum.php b/src/enums/RelationshipTypeEnum.php
new file mode 100644
index 0000000..f696bd4
--- /dev/null
+++ b/src/enums/RelationshipTypeEnum.php
@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace alsvanzelf\jsonapi\enums;
+
+enum RelationshipTypeEnum {
+	case ToOne;
+	case ToMany;
+}
diff --git a/src/enums/SortOrderEnum.php b/src/enums/SortOrderEnum.php
new file mode 100644
index 0000000..8995d47
--- /dev/null
+++ b/src/enums/SortOrderEnum.php
@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace alsvanzelf\jsonapi\enums;
+
+enum SortOrderEnum {
+	case Ascending;
+	case Descending;
+}
diff --git a/src/exceptions/DuplicateException.php b/src/exceptions/DuplicateException.php
index c2a2d6f..a1c5c9a 100644
--- a/src/exceptions/DuplicateException.php
+++ b/src/exceptions/DuplicateException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\exceptions;
 
 use alsvanzelf\jsonapi\exceptions\Exception;
diff --git a/src/exceptions/Exception.php b/src/exceptions/Exception.php
index d245a9f..5318984 100644
--- a/src/exceptions/Exception.php
+++ b/src/exceptions/Exception.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\exceptions;
 
 class Exception extends \Exception {}
diff --git a/src/exceptions/InputException.php b/src/exceptions/InputException.php
index 8f4d929..ce121c1 100644
--- a/src/exceptions/InputException.php
+++ b/src/exceptions/InputException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\exceptions;
 
 use alsvanzelf\jsonapi\exceptions\Exception;
diff --git a/src/extensions/AtomicOperationsDocument.php b/src/extensions/AtomicOperationsDocument.php
index fc16caf..0ece144 100644
--- a/src/extensions/AtomicOperationsDocument.php
+++ b/src/extensions/AtomicOperationsDocument.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\extensions;
 
 use alsvanzelf\jsonapi\Document;
diff --git a/src/extensions/AtomicOperationsExtension.php b/src/extensions/AtomicOperationsExtension.php
index 6b6cffd..e2d1d42 100644
--- a/src/extensions/AtomicOperationsExtension.php
+++ b/src/extensions/AtomicOperationsExtension.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\extensions;
 
 use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
diff --git a/src/helpers/AtMemberManager.php b/src/helpers/AtMemberManager.php
index 78cbf3e..87272b9 100644
--- a/src/helpers/AtMemberManager.php
+++ b/src/helpers/AtMemberManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\helpers;
 
 use alsvanzelf\jsonapi\helpers\Converter;
diff --git a/src/helpers/Converter.php b/src/helpers/Converter.php
index 336d4a8..ef09203 100644
--- a/src/helpers/Converter.php
+++ b/src/helpers/Converter.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\helpers;
 
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
 use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 use alsvanzelf\jsonapi\interfaces\ProfileInterface;
@@ -37,12 +40,13 @@ public static function camelCaseToWords($camelCase) {
 	/**
 	 * generates the value for a content type header, with extensions and profiles merged in if available
 	 * 
-	 * @param  string               $contentType
 	 * @param  ExtensionInterface[] $extensions
 	 * @param  ProfileInterface[]   $profiles
 	 * @return string
 	 */
-	public static function prepareContentType($contentType, array $extensions, array $profiles) {
+	public static function prepareContentType(ContentTypeEnum $contentType, array $extensions, array $profiles) {
+		$contentType = $contentType->value;
+		
 		if ($extensions !== []) {
 			$extensionLinks = [];
 			foreach ($extensions as $extension) {
diff --git a/src/helpers/ExtensionMemberManager.php b/src/helpers/ExtensionMemberManager.php
index 1bfd257..f971fc8 100644
--- a/src/helpers/ExtensionMemberManager.php
+++ b/src/helpers/ExtensionMemberManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\helpers;
 
 use alsvanzelf\jsonapi\helpers\Converter;
diff --git a/src/helpers/HttpStatusCodeManager.php b/src/helpers/HttpStatusCodeManager.php
index 7c051b4..857f7e7 100644
--- a/src/helpers/HttpStatusCodeManager.php
+++ b/src/helpers/HttpStatusCodeManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\helpers;
 
 use alsvanzelf\jsonapi\exceptions\InputException;
diff --git a/src/helpers/LinksManager.php b/src/helpers/LinksManager.php
index 0fa10c9..844ba9f 100644
--- a/src/helpers/LinksManager.php
+++ b/src/helpers/LinksManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\helpers;
 
 use alsvanzelf\jsonapi\objects\LinkObject;
diff --git a/src/helpers/RequestParser.php b/src/helpers/RequestParser.php
index b672ff7..73f63bd 100644
--- a/src/helpers/RequestParser.php
+++ b/src/helpers/RequestParser.php
@@ -1,15 +1,15 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\helpers;
 
-use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
+use alsvanzelf\jsonapi\enums\SortOrderEnum;
 use Psr\Http\Message\RequestInterface;
 use Psr\Http\Message\ServerRequestInterface;
 
 class RequestParser {
-	const SORT_ASCENDING  = 'ascending';
-	const SORT_DESCENDING = 'descending';
-	
 	/** @var array */
 	protected static $defaults = [
 		/**
@@ -49,8 +49,8 @@ public static function fromSuperglobals() {
 		
 		$document = $_POST;
 		if ($document === [] && isset($_SERVER['CONTENT_TYPE'])) {
-			$documentIsJsonapi = (str_contains((string) $_SERVER['CONTENT_TYPE'], Document::CONTENT_TYPE_OFFICIAL));
-			$documentIsJson    = (str_contains((string) $_SERVER['CONTENT_TYPE'], Document::CONTENT_TYPE_DEBUG));
+			$documentIsJsonapi = (str_contains((string) $_SERVER['CONTENT_TYPE'], ContentTypeEnum::Official->value));
+			$documentIsJson    = (str_contains((string) $_SERVER['CONTENT_TYPE'], ContentTypeEnum::Debug->value));
 			
 			if ($documentIsJsonapi || $documentIsJson) {
 				$document = json_decode(file_get_contents('php://input'), true);
@@ -186,7 +186,7 @@ public function hasSortFields() {
 	 * @param  array $options optional {@see RequestParser::$defaults}
 	 * @return string[]|array<array{
 	 *         field: string, // the sort field, without any minus sign for descending sort order
-	 *         order: string, // one of the RequestParser::SORT_* constants
+	 *         order: SortOrderEnum,
 	 * }>
 	 */
 	public function getSortFields(array $options=[]) {
@@ -203,11 +203,11 @@ public function getSortFields(array $options=[]) {
 		
 		$sort = [];
 		foreach ($fields as $field) {
-			$order = RequestParser::SORT_ASCENDING;
+			$order = SortOrderEnum::Ascending;
 			
 			if (str_starts_with($field, '-')) {
 				$field = substr($field, 1);
-				$order = RequestParser::SORT_DESCENDING;
+				$order = SortOrderEnum::Descending;
 			}
 			
 			$sort[] = [
diff --git a/src/helpers/Validator.php b/src/helpers/Validator.php
index 2c9e573..a31d1d5 100644
--- a/src/helpers/Validator.php
+++ b/src/helpers/Validator.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\helpers;
 
+use alsvanzelf\jsonapi\enums\ObjectContainerEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\interfaces\ResourceInterface;
@@ -10,12 +13,6 @@
  * @internal
  */
 class Validator {
-	const OBJECT_CONTAINER_TYPE          = 'type';
-	const OBJECT_CONTAINER_ID            = 'id';
-	const OBJECT_CONTAINER_LID           = 'lid';
-	const OBJECT_CONTAINER_ATTRIBUTES    = 'attributes';
-	const OBJECT_CONTAINER_RELATIONSHIPS = 'relationships';
-	
 	/** @var array */
 	protected $usedFields = [];
 	/** @var array */
@@ -36,12 +33,11 @@ class Validator {
 	 * @see https://jsonapi.org/format/1.1/#document-resource-object-fields
 	 * 
 	 * @param  string[] $fieldNames
-	 * @param  string   $objectContainer one of the Validator::OBJECT_CONTAINER_* constants
 	 * @param  array    $options         optional {@see Validator::$defaults}
 	 * 
 	 * @throws DuplicateException
 	 */
-	public function claimUsedFields(array $fieldNames, $objectContainer, array $options=[]) {
+	public function claimUsedFields(array $fieldNames, ObjectContainerEnum $objectContainer, array $options=[]) {
 		$options = array_merge(self::$defaults, $options);
 		
 		foreach ($fieldNames as $fieldName) {
@@ -56,18 +52,15 @@ public function claimUsedFields(array $fieldNames, $objectContainer, array $opti
 			/**
 			 * @note this is not allowed by the specification
 			 */
-			if ($this->usedFields[$fieldName] === Validator::OBJECT_CONTAINER_TYPE && $options['enforceTypeFieldNamespace'] === false) {
+			if ($this->usedFields[$fieldName] === ObjectContainerEnum::Type && $options['enforceTypeFieldNamespace'] === false) {
 				continue;
 			}
 			
-			throw new DuplicateException('field name "'.$fieldName.'" already in use at "data.'.$this->usedFields[$fieldName].'"');
+			throw new DuplicateException('field name "'.$fieldName.'" already in use at "data.'.$this->usedFields[$fieldName]->value.'"');
 		}
 	}
 	
-	/**
-	 * @param string $objectContainerToClear one of the Validator::OBJECT_CONTAINER_* constants
-	 */
-	public function clearUsedFields($objectContainerToClear) {
+	public function clearUsedFields(ObjectContainerEnum $objectContainerToClear) {
 		foreach ($this->usedFields as $fieldName => $containerFound) {
 			if ($containerFound !== $objectContainerToClear) {
 				continue;
diff --git a/src/interfaces/DocumentInterface.php b/src/interfaces/DocumentInterface.php
index 6d3e348..b776845 100644
--- a/src/interfaces/DocumentInterface.php
+++ b/src/interfaces/DocumentInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 use alsvanzelf\jsonapi\exceptions\Exception;
diff --git a/src/interfaces/ExtensionInterface.php b/src/interfaces/ExtensionInterface.php
index 3c07497..57242b0 100644
--- a/src/interfaces/ExtensionInterface.php
+++ b/src/interfaces/ExtensionInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 interface ExtensionInterface {
diff --git a/src/interfaces/ObjectInterface.php b/src/interfaces/ObjectInterface.php
index 7658540..9766f83 100644
--- a/src/interfaces/ObjectInterface.php
+++ b/src/interfaces/ObjectInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 interface ObjectInterface {
diff --git a/src/interfaces/PaginableInterface.php b/src/interfaces/PaginableInterface.php
index 2068244..32f4b77 100644
--- a/src/interfaces/PaginableInterface.php
+++ b/src/interfaces/PaginableInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 interface PaginableInterface {
diff --git a/src/interfaces/ProfileInterface.php b/src/interfaces/ProfileInterface.php
index b4a6f2f..5f2f654 100644
--- a/src/interfaces/ProfileInterface.php
+++ b/src/interfaces/ProfileInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 interface ProfileInterface {
diff --git a/src/interfaces/RecursiveResourceContainerInterface.php b/src/interfaces/RecursiveResourceContainerInterface.php
index 93c81f1..c77041f 100644
--- a/src/interfaces/RecursiveResourceContainerInterface.php
+++ b/src/interfaces/RecursiveResourceContainerInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 use alsvanzelf\jsonapi\objects\ResourceObject;
diff --git a/src/interfaces/ResourceContainerInterface.php b/src/interfaces/ResourceContainerInterface.php
index ec50ac8..6e136ca 100644
--- a/src/interfaces/ResourceContainerInterface.php
+++ b/src/interfaces/ResourceContainerInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 use alsvanzelf\jsonapi\objects\ResourceIdentifierObject;
diff --git a/src/interfaces/ResourceInterface.php b/src/interfaces/ResourceInterface.php
index 281d80d..65f202c 100644
--- a/src/interfaces/ResourceInterface.php
+++ b/src/interfaces/ResourceInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\interfaces;
 
 use alsvanzelf\jsonapi\objects\ResourceIdentifierObject;
diff --git a/src/objects/AttributesObject.php b/src/objects/AttributesObject.php
index 8947110..c90f989 100644
--- a/src/objects/AttributesObject.php
+++ b/src/objects/AttributesObject.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\helpers\Converter;
diff --git a/src/objects/ErrorObject.php b/src/objects/ErrorObject.php
index 92e3f17..d8e5551 100644
--- a/src/objects/ErrorObject.php
+++ b/src/objects/ErrorObject.php
@@ -1,9 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\Document;
-use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\helpers\Converter;
 use alsvanzelf\jsonapi\helpers\HttpStatusCodeManager;
 use alsvanzelf\jsonapi\helpers\LinksManager;
diff --git a/src/objects/JsonapiObject.php b/src/objects/JsonapiObject.php
index 3d581d9..a9ff8d9 100644
--- a/src/objects/JsonapiObject.php
+++ b/src/objects/JsonapiObject.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\enums\JsonapiVersionEnum;
 use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
 use alsvanzelf\jsonapi\interfaces\HasMetaInterface;
 use alsvanzelf\jsonapi\interfaces\ProfileInterface;
@@ -10,7 +13,7 @@
 use alsvanzelf\jsonapi\objects\MetaObject;
 
 class JsonapiObject extends AbstractObject implements HasMetaInterface {
-	/** @var string */
+	/** @var JsonapiVersionEnum */
 	protected $version;
 	/** @var ExtensionInterface[] */
 	protected $extensions = [];
@@ -19,10 +22,7 @@ class JsonapiObject extends AbstractObject implements HasMetaInterface {
 	/** @var MetaObject */
 	protected $meta;
 	
-	/**
-	 * @param ?string $version one of the Document::JSONAPI_VERSION_* constants, optional, defaults to Document::JSONAPI_VERSION_LATEST
-	 */
-	public function __construct($version=Document::JSONAPI_VERSION_LATEST) {
+	public function __construct(?JsonapiVersionEnum $version=JsonapiVersionEnum::Latest) {
 		if ($version !== null) {
 			$this->setVersion($version);
 		}
@@ -48,10 +48,7 @@ public function addMeta($key, $value) {
 	 * spec api
 	 */
 	
-	/**
-	 * @param string $version
-	 */
-	public function setVersion($version) {
+	public function setVersion(JsonapiVersionEnum $version) {
 		$this->version = $version;
 	}
 	
@@ -113,7 +110,7 @@ public function toArray() {
 			$array = array_merge($array, $this->getExtensionMembers());
 		}
 		if ($this->version !== null) {
-			$array['version'] = $this->version;
+			$array['version'] = $this->version->value;
 		}
 		if ($this->extensions !== []) {
 			$array['ext'] = [];
diff --git a/src/objects/LinkObject.php b/src/objects/LinkObject.php
index e7a06e0..9e6286e 100644
--- a/src/objects/LinkObject.php
+++ b/src/objects/LinkObject.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\interfaces\HasMetaInterface;
diff --git a/src/objects/LinksObject.php b/src/objects/LinksObject.php
index bad6155..aa178af 100644
--- a/src/objects/LinksObject.php
+++ b/src/objects/LinksObject.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
diff --git a/src/objects/MetaObject.php b/src/objects/MetaObject.php
index e98a2c9..bee46b2 100644
--- a/src/objects/MetaObject.php
+++ b/src/objects/MetaObject.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\helpers\Converter;
diff --git a/src/objects/RelationshipObject.php b/src/objects/RelationshipObject.php
index 6cb0efd..af2202e 100644
--- a/src/objects/RelationshipObject.php
+++ b/src/objects/RelationshipObject.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\CollectionDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\helpers\LinksManager;
 use alsvanzelf\jsonapi\interfaces\HasLinksInterface;
@@ -18,30 +21,16 @@
 class RelationshipObject extends AbstractObject implements PaginableInterface, RecursiveResourceContainerInterface, HasLinksInterface, HasMetaInterface {
 	use LinksManager;
 	
-	const TO_ONE  = 'one';
-	const TO_MANY = 'many';
-	
 	/** @var MetaObject */
 	protected $meta;
 	/** @var ResourceInterface */
 	protected $resource;
-	/** @var string one of the RelationshipObject::TO_* constants */
-	protected $type;
 	/** @var ResourceInterface[] */
 	protected $resources = [];
 	
-	/**
-	 * @param string $type one of the RelationshipObject::TO_* constants
-	 * 
-	 * @throws InputException if $type is unknown
-	 */
-	public function __construct($type) {
-		if (in_array($type, [RelationshipObject::TO_ONE, RelationshipObject::TO_MANY], $strict=true) === false) {
-			throw new InputException('unknown relationship type "'.$type.'"');
-		}
-		
-		$this->type = $type;
-	}
+	public function __construct(
+		protected RelationshipTypeEnum $type,
+	) {}
 	
 	/**
 	 * human api
@@ -69,7 +58,7 @@ public static function fromAnything($relation, array $links=[], array $meta=[])
 			$relationshipObject = self::fromCollectionDocument($relation, $links, $meta);
 		}
 		elseif ($relation === null) {
-			$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+			$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		}
 		else {
 			throw new InputException('unknown format of relation "'.gettype($relation).'"');
@@ -82,18 +71,15 @@ public static function fromAnything($relation, array $links=[], array $meta=[])
 	 * @param  ResourceInterface $resource
 	 * @param  array             $links    optional
 	 * @param  array             $meta     optional
-	 * @param  string            $type     optional, one of the RelationshipObject::TO_* constants, defaults to RelationshipObject::TO_ONE
 	 * @return RelationshipObject
 	 */
-	public static function fromResource(ResourceInterface $resource, array $links=[], array $meta=[], $type=RelationshipObject::TO_ONE) {
+	public static function fromResource(ResourceInterface $resource, array $links=[], array $meta=[], RelationshipTypeEnum $type=RelationshipTypeEnum::ToOne) {
 		$relationshipObject = new self($type);
 		
-		if ($type === RelationshipObject::TO_ONE) {
-			$relationshipObject->setResource($resource);
-		}
-		elseif ($type === RelationshipObject::TO_MANY) {
-			$relationshipObject->addResource($resource);
-		}
+		match ($type) {
+			RelationshipTypeEnum::ToOne  => $relationshipObject->setResource($resource),
+			RelationshipTypeEnum::ToMany => $relationshipObject->addResource($resource),
+		};
 		
 		if ($links !== []) {
 			$relationshipObject->setLinksObject(LinksObject::fromArray($links));
@@ -112,7 +98,7 @@ public static function fromResource(ResourceInterface $resource, array $links=[]
 	 * @return RelationshipObject
 	 */
 	public static function fromCollectionDocument(CollectionDocument $collectionDocument, array $links=[], array $meta=[]) {
-		$relationshipObject = new self(RelationshipObject::TO_MANY);
+		$relationshipObject = new self(RelationshipTypeEnum::ToMany);
 		
 		foreach ($collectionDocument->getContainedResources() as $resource) {
 			$relationshipObject->addResource($resource);
@@ -148,7 +134,7 @@ public function setRelatedLink($href, array $meta=[]) {
 	 * @throws InputException if used on a to-one relationship
 	 */
 	public function setPaginationLinks($previousHref=null, $nextHref=null, $firstHref=null, $lastHref=null) {
-		if ($this->type === RelationshipObject::TO_ONE) {
+		if ($this->type === RelationshipTypeEnum::ToOne) {
 			throw new InputException('can not add pagination links to a to-one relationship');
 		}
 		
@@ -190,7 +176,7 @@ public function addMeta($key, $value) {
 	 * @throws InputException if used on a to-many relationship, use {@see ->addResource()} instead
 	 */
 	public function setResource(ResourceInterface $resource) {
-		if ($this->type === RelationshipObject::TO_MANY) {
+		if ($this->type === RelationshipTypeEnum::ToMany) {
 			throw new InputException('can not set a resource on a to-many relationship, use ->addResource()');
 		}
 		
@@ -205,7 +191,7 @@ public function setResource(ResourceInterface $resource) {
 	 * @throws InputException if used on a to-one relationship, use {@see ->setResource()} instead
 	 */
 	public function addResource(ResourceInterface $resource) {
-		if ($this->type === RelationshipObject::TO_ONE) {
+		if ($this->type === RelationshipTypeEnum::ToOne) {
 			throw new InputException('can not add a resource to a to-one relationship, use ->setResource()');
 		}
 		
@@ -235,18 +221,20 @@ public function hasResource(ResourceInterface $otherResource) {
 		if ($this->isEmpty()) {
 			return false;
 		}
-		if ($this->type === RelationshipObject::TO_ONE) {
-			return $this->resource->getResource()->equals($otherResource->getResource());
-		}
-		if ($this->type === RelationshipObject::TO_MANY) {
-			foreach ($this->resources as $ownResource) {
-				if ($ownResource->getResource()->equals($otherResource->getResource())) {
-					return true;
+		
+		switch ($this->type) {
+			case RelationshipTypeEnum::ToOne:
+				return $this->resource->getResource()->equals($otherResource->getResource());
+			
+			case RelationshipTypeEnum::ToMany:
+				foreach ($this->resources as $ownResource) {
+					if ($ownResource->getResource()->equals($otherResource->getResource())) {
+						return true;
+					}
 				}
-			}
+				
+				return false;
 		}
-		
-		return false;
 	}
 	
 	/**
@@ -254,10 +242,10 @@ public function hasResource(ResourceInterface $otherResource) {
 	 */
 	
 	public function isEmpty() {
-		if ($this->type === RelationshipObject::TO_ONE && $this->resource !== null) {
+		if ($this->type === RelationshipTypeEnum::ToOne && $this->resource !== null) {
 			return false;
 		}
-		if ($this->type === RelationshipObject::TO_MANY && $this->resources !== []) {
+		if ($this->type === RelationshipTypeEnum::ToMany && $this->resources !== []) {
 			return false;
 		}
 		if ($this->links !== null && $this->links->isEmpty() === false) {
@@ -289,13 +277,13 @@ public function toArray() {
 		if ($this->links !== null && $this->links->isEmpty() === false) {
 			$array['links'] = $this->links->toArray();
 		}
-		if ($this->type === RelationshipObject::TO_ONE) {
+		if ($this->type === RelationshipTypeEnum::ToOne) {
 			$array['data'] = null;
 			if ($this->resource !== null) {
 				$array['data'] = $this->resource->getResource($identifierOnly=true)->toArray();
 			}
 		}
-		if ($this->type === RelationshipObject::TO_MANY) {
+		if ($this->type === RelationshipTypeEnum::ToMany) {
 			$array['data'] = [];
 			foreach ($this->resources as $resource) {
 				$array['data'][] = $resource->getResource($identifierOnly=true)->toArray();
@@ -317,7 +305,7 @@ public function getNestedContainedResourceObjects() {
 			return [];
 		}
 		
-		$resources       = ($this->type === RelationshipObject::TO_ONE) ? [$this->resource] : $this->resources;
+		$resources       = ($this->type === RelationshipTypeEnum::ToOne) ? [$this->resource] : $this->resources;
 		$resourceObjects = [];
 		
 		foreach ($resources as $resource) {
diff --git a/src/objects/RelationshipsObject.php b/src/objects/RelationshipsObject.php
index 8c8d0be..a9a684e 100644
--- a/src/objects/RelationshipsObject.php
+++ b/src/objects/RelationshipsObject.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
diff --git a/src/objects/ResourceIdentifierObject.php b/src/objects/ResourceIdentifierObject.php
index 715456d..d9d12e7 100644
--- a/src/objects/ResourceIdentifierObject.php
+++ b/src/objects/ResourceIdentifierObject.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
+use alsvanzelf\jsonapi\enums\ObjectContainerEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\helpers\Validator;
@@ -41,9 +44,9 @@ public function __construct($type=null, $id=null) {
 		}
 		
 		// always mark as used, as these keys are reserved
-		$this->validator->claimUsedFields($fieldNames=['type'], Validator::OBJECT_CONTAINER_TYPE);
-		$this->validator->claimUsedFields($fieldNames=['id'], Validator::OBJECT_CONTAINER_ID);
-		$this->validator->claimUsedFields($fieldNames=['lid'], Validator::OBJECT_CONTAINER_LID);
+		$this->validator->claimUsedFields($fieldNames=['type'], ObjectContainerEnum::Type);
+		$this->validator->claimUsedFields($fieldNames=['id'], ObjectContainerEnum::Id);
+		$this->validator->claimUsedFields($fieldNames=['lid'], ObjectContainerEnum::Lid);
 	}
 	
 	/**
diff --git a/src/objects/ResourceObject.php b/src/objects/ResourceObject.php
index e0d59d3..eff9d30 100644
--- a/src/objects/ResourceObject.php
+++ b/src/objects/ResourceObject.php
@@ -1,12 +1,14 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\CollectionDocument;
+use alsvanzelf\jsonapi\enums\ObjectContainerEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\helpers\Converter;
 use alsvanzelf\jsonapi\helpers\LinksManager;
-use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\HasAttributesInterface;
 use alsvanzelf\jsonapi\interfaces\HasLinksInterface;
 use alsvanzelf\jsonapi\interfaces\RecursiveResourceContainerInterface;
@@ -90,7 +92,7 @@ public function add($key, $value, array $options=[]) {
 			$this->attributes = new AttributesObject();
 		}
 		
-		$this->validator->claimUsedFields([$key], Validator::OBJECT_CONTAINER_ATTRIBUTES, $options);
+		$this->validator->claimUsedFields([$key], ObjectContainerEnum::Attributes, $options);
 		
 		$this->attributes->add($key, $value);
 	}
@@ -129,8 +131,8 @@ public function setSelfLink($href, array $meta=[]) {
 	 */
 	public function setAttributesObject(AttributesObject $attributesObject, array $options=[]) {
 		$newKeys = $attributesObject->getKeys();
-		$this->validator->clearUsedFields(Validator::OBJECT_CONTAINER_ATTRIBUTES);
-		$this->validator->claimUsedFields($newKeys, Validator::OBJECT_CONTAINER_ATTRIBUTES, $options);
+		$this->validator->clearUsedFields(ObjectContainerEnum::Attributes);
+		$this->validator->claimUsedFields($newKeys, ObjectContainerEnum::Attributes, $options);
 		
 		$this->attributes = $attributesObject;
 	}
@@ -151,7 +153,7 @@ public function addRelationshipObject($key, RelationshipObject $relationshipObje
 			$this->setRelationshipsObject(new RelationshipsObject());
 		}
 		
-		$this->validator->claimUsedFields([$key], Validator::OBJECT_CONTAINER_RELATIONSHIPS, $options);
+		$this->validator->claimUsedFields([$key], ObjectContainerEnum::Relationships, $options);
 		
 		$this->relationships->addRelationshipObject($key, $relationshipObject);
 	}
@@ -161,8 +163,8 @@ public function addRelationshipObject($key, RelationshipObject $relationshipObje
 	 */
 	public function setRelationshipsObject(RelationshipsObject $relationshipsObject) {
 		$newKeys = $relationshipsObject->getKeys();
-		$this->validator->clearUsedFields(Validator::OBJECT_CONTAINER_RELATIONSHIPS);
-		$this->validator->claimUsedFields($newKeys, Validator::OBJECT_CONTAINER_RELATIONSHIPS);
+		$this->validator->clearUsedFields(ObjectContainerEnum::Relationships);
+		$this->validator->claimUsedFields($newKeys, ObjectContainerEnum::Relationships);
 		
 		$this->relationships = $relationshipsObject;
 	}
diff --git a/src/profiles/CursorPaginationProfile.php b/src/profiles/CursorPaginationProfile.php
index 8ab4a9f..5140cb7 100644
--- a/src/profiles/CursorPaginationProfile.php
+++ b/src/profiles/CursorPaginationProfile.php
@@ -1,9 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapi\profiles;
 
-use alsvanzelf\jsonapi\Document;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\interfaces\HasLinksInterface;
 use alsvanzelf\jsonapi\interfaces\HasMetaInterface;
@@ -202,7 +204,7 @@ public function setItemMeta(ResourceInterface $resource, $cursor) {
 		];
 		
 		if ($resource instanceof ResourceDocument) {
-			$resource->addMeta('page', $metadata, $level=Document::LEVEL_RESOURCE);
+			$resource->addMeta('page', $metadata, $level=DocumentLevelEnum::Resource);
 		}
 		else {
 			$resource->addMeta('page', $metadata);
diff --git a/tests/CollectionDocumentTest.php b/tests/CollectionDocumentTest.php
index fd255f0..97ce3e8 100644
--- a/tests/CollectionDocumentTest.php
+++ b/tests/CollectionDocumentTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
 use PHPUnit\Framework\Attributes\DataProvider;
diff --git a/tests/ConverterTest.php b/tests/ConverterTest.php
index 6eef343..57f41b6 100644
--- a/tests/ConverterTest.php
+++ b/tests/ConverterTest.php
@@ -1,11 +1,14 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
 use PHPUnit\Framework\Attributes\DataProvider;
 use PHPUnit\Framework\TestCase;
 use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use alsvanzelf\jsonapiTests\profiles\TestProfile;
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
 use alsvanzelf\jsonapi\helpers\Converter;
 use alsvanzelf\jsonapi\objects\AttributesObject;
 
@@ -68,7 +71,7 @@ public static function dataProviderCamelCaseToWords_HappyPath() {
 	 * @group Profiles
 	 */
 	public function testPrepareContentType_HappyPath() {
-		$this->assertSame('foo', Converter::prepareContentType('foo', [], []));
+		$this->assertSame(ContentTypeEnum::Official->value, Converter::prepareContentType(ContentTypeEnum::Official, [], []));
 	}
 	
 	/**
@@ -78,7 +81,7 @@ public function testPrepareContentType_WithExtensionStringLink() {
 		$extension = new TestExtension();
 		$extension->setOfficialLink('bar');
 		
-		$this->assertSame('foo; ext="bar"', Converter::prepareContentType('foo', [$extension], []));
+		$this->assertSame(ContentTypeEnum::Official->value.'; ext="bar"', Converter::prepareContentType(ContentTypeEnum::Official, [$extension], []));
 	}
 	
 	/**
@@ -88,7 +91,7 @@ public function testPrepareContentType_WithProfileStringLink() {
 		$profile = new TestProfile();
 		$profile->setOfficialLink('bar');
 		
-		$this->assertSame('foo; profile="bar"', Converter::prepareContentType('foo', [], [$profile]));
+		$this->assertSame(ContentTypeEnum::Official->value.'; profile="bar"', Converter::prepareContentType(ContentTypeEnum::Official, [], [$profile]));
 	}
 	
 	/**
@@ -108,7 +111,7 @@ public function testPrepareContentType_WithMultipleExtensionsAndProfiles() {
 		$profile2 = new TestProfile();
 		$profile2->setOfficialLink('baz');
 		
-		$this->assertSame('foo; ext="bar baz"; profile="bar baz"', Converter::prepareContentType('foo', [$extension1, $extension2], [$profile1, $profile2]));
+		$this->assertSame(ContentTypeEnum::Official->value.'; ext="bar baz"; profile="bar baz"', Converter::prepareContentType(ContentTypeEnum::Official, [$extension1, $extension2], [$profile1, $profile2]));
 	}
 }
 
diff --git a/tests/DocumentTest.php b/tests/DocumentTest.php
index 6e8e51c..024f989 100644
--- a/tests/DocumentTest.php
+++ b/tests/DocumentTest.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\InputException;
@@ -68,7 +71,7 @@ public function testAddLink_BlocksJsonapiLevel() {
 		$this->expectException(InputException::class);
 		$this->expectExceptionMessage('level "jsonapi" can not be used for links');
 		
-		$document->addLink('foo', 'https://jsonapi.org', $meta=[], $level=Document::LEVEL_JSONAPI);
+		$document->addLink('foo', 'https://jsonapi.org', $meta=[], $level=DocumentLevelEnum::Jsonapi);
 	}
 	
 	public function testAddLink_BlocksResourceLevel() {
@@ -77,16 +80,7 @@ public function testAddLink_BlocksResourceLevel() {
 		$this->expectException(InputException::class);
 		$this->expectExceptionMessage('level "resource" can only be set on a ResourceDocument');
 		
-		$document->addLink('foo', 'https://jsonapi.org', $meta=[], $level=Document::LEVEL_RESOURCE);
-	}
-	
-	public function testAddLink_BlocksUnknownLevel() {
-		$document = new Document();
-		
-		$this->expectException(InputException::class);
-		$this->expectExceptionMessage('unknown level "foo"');
-		
-		$document->addLink('foo', 'https://jsonapi.org', $meta=[], $level='foo');
+		$document->addLink('foo', 'https://jsonapi.org', $meta=[], $level=DocumentLevelEnum::Resource);
 	}
 	
 	public function testSetSelfLink_HappyPath() {
@@ -160,7 +154,7 @@ public function testAddMeta_AtJsonapiLevel() {
 		$this->assertArrayHasKey('jsonapi', $array);
 		$this->assertArrayNotHasKey('meta', $array['jsonapi']);
 		
-		$document->addMeta('foo', 'bar', $level=Document::LEVEL_JSONAPI);
+		$document->addMeta('foo', 'bar', $level=DocumentLevelEnum::Jsonapi);
 		
 		$array = $document->toArray();
 		
@@ -178,16 +172,7 @@ public function testAddMeta_BlocksResourceLevel() {
 		$this->expectException(InputException::class);
 		$this->expectExceptionMessage('level "resource" can only be set on a ResourceDocument');
 		
-		$document->addMeta('foo', 'bar', $level=Document::LEVEL_RESOURCE);
-	}
-	
-	public function testAddMeta_BlocksUnknownLevel() {
-		$document = new Document();
-		
-		$this->expectException(InputException::class);
-		$this->expectExceptionMessage('unknown level "foo"');
-		
-		$document->addMeta('foo', 'bar', $level='foo');
+		$document->addMeta('foo', 'bar', $level=DocumentLevelEnum::Resource);
 	}
 	
 	public function testAddLinkObject_HappyPath() {
diff --git a/tests/ErrorsDocumentTest.php b/tests/ErrorsDocumentTest.php
index 151f44e..10e694f 100644
--- a/tests/ErrorsDocumentTest.php
+++ b/tests/ErrorsDocumentTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
 use PHPUnit\Framework\Attributes\DataProvider;
diff --git a/tests/ExampleOutputTest.php b/tests/ExampleOutputTest.php
index 4ccd0c9..20f641a 100644
--- a/tests/ExampleOutputTest.php
+++ b/tests/ExampleOutputTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
 use PHPUnit\Framework\Attributes\DataProvider;
diff --git a/tests/MetaDocumentTest.php b/tests/MetaDocumentTest.php
index e0daee4..cfb6d18 100644
--- a/tests/MetaDocumentTest.php
+++ b/tests/MetaDocumentTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
 use alsvanzelf\jsonapi\MetaDocument;
diff --git a/tests/ResourceDocumentTest.php b/tests/ResourceDocumentTest.php
index 36bcf7e..01da41d 100644
--- a/tests/ResourceDocumentTest.php
+++ b/tests/ResourceDocumentTest.php
@@ -1,9 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
-use alsvanzelf\jsonapi\Document;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\objects\AttributesObject;
@@ -86,9 +88,9 @@ public function testAddRelationship_DoNotIncludeContainedResources() {
 	
 	public function testAddMeta_HappyPath() {
 		$document = new ResourceDocument();
-		$document->addMeta('foo', 'root', $level=Document::LEVEL_ROOT);
-		$document->addMeta('bar', 'resource', $level=Document::LEVEL_RESOURCE);
-		$document->addMeta('baz', 'jsonapi', $level=Document::LEVEL_JSONAPI);
+		$document->addMeta('foo', 'root', $level=DocumentLevelEnum::Root);
+		$document->addMeta('bar', 'resource', $level=DocumentLevelEnum::Resource);
+		$document->addMeta('baz', 'jsonapi', $level=DocumentLevelEnum::Jsonapi);
 		
 		$array = $document->toArray();
 		
@@ -122,7 +124,7 @@ public function testAddMeta_RecreateJsonapiObject() {
 		
 		$this->assertArrayNotHasKey('jsonapi', $array);
 		
-		$document->addMeta('baz', 'jsonapi', $level=Document::LEVEL_JSONAPI);
+		$document->addMeta('baz', 'jsonapi', $level=DocumentLevelEnum::Jsonapi);
 		
 		$array = $document->toArray();
 		
diff --git a/tests/SeparateProcessTest.php b/tests/SeparateProcessTest.php
index ec00ccd..95f8290 100644
--- a/tests/SeparateProcessTest.php
+++ b/tests/SeparateProcessTest.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
-use alsvanzelf\jsonapiTests\TestableNonAbstractDocument as Document;
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
+use alsvanzelf\jsonapiTests\TestableNonAbstractDocument;
 use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use alsvanzelf\jsonapiTests\profiles\TestProfile;
 use PHPUnit\Framework\TestCase;
@@ -15,7 +18,7 @@ class SeparateProcessTest extends TestCase {
 	 * @runInSeparateProcess
 	 */
 	public function testSendResponse_HappyPath() {
-		$document = new Document();
+		$document = new TestableNonAbstractDocument();
 		
 		ob_start();
 		$document->sendResponse();
@@ -28,7 +31,7 @@ public function testSendResponse_HappyPath() {
 	 * @runInSeparateProcess
 	 */
 	public function testSendResponse_NoContent() {
-		$document = new Document();
+		$document = new TestableNonAbstractDocument();
 		$document->setHttpStatusCode(204);
 		
 		ob_start();
@@ -47,30 +50,30 @@ public function testSendResponse_ContentTypeHeader() {
 			$this->markTestSkipped('can not run without xdebug');
 		}
 		
-		$document = new Document();
+		$document = new TestableNonAbstractDocument();
 		
 		ob_start();
 		$document->sendResponse();
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Official->value], xdebug_get_headers());
 		
-		$options = ['contentType' => Document::CONTENT_TYPE_OFFICIAL];
+		$options = ['contentType' => ContentTypeEnum::Official];
 		ob_start();
 		$document->sendResponse($options);
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Official->value], xdebug_get_headers());
 		
-		$options = ['contentType' => Document::CONTENT_TYPE_DEBUG];
+		$options = ['contentType' => ContentTypeEnum::Debug];
 		ob_start();
 		$document->sendResponse($options);
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_DEBUG], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Debug->value], xdebug_get_headers());
 		
-		$options = ['contentType' => Document::CONTENT_TYPE_JSONP];
+		$options = ['contentType' => ContentTypeEnum::Jsonp];
 		ob_start();
 		$document->sendResponse($options);
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_JSONP], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Jsonp->value], xdebug_get_headers());
 	}
 	
 	/**
@@ -86,13 +89,13 @@ public function testSendResponse_ContentTypeHeaderWithExtensions() {
 		$extension->setNamespace('one');
 		$extension->setOfficialLink('https://jsonapi.org');
 		
-		$document = new Document();
+		$document = new TestableNonAbstractDocument();
 		$document->applyExtension($extension);
 		
 		ob_start();
 		$document->sendResponse();
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; ext="https://jsonapi.org"'], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Official->value.'; ext="https://jsonapi.org"'], xdebug_get_headers());
 		
 		$extension = new TestExtension();
 		$extension->setNamespace('two');
@@ -102,7 +105,7 @@ public function testSendResponse_ContentTypeHeaderWithExtensions() {
 		ob_start();
 		$document->sendResponse();
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; ext="https://jsonapi.org https://jsonapi.org/2"'], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Official->value.'; ext="https://jsonapi.org https://jsonapi.org/2"'], xdebug_get_headers());
 	}
 	
 	/**
@@ -117,13 +120,13 @@ public function testSendResponse_ContentTypeHeaderWithProfiles() {
 		$profile = new TestProfile();
 		$profile->setOfficialLink('https://jsonapi.org');
 		
-		$document = new Document();
+		$document = new TestableNonAbstractDocument();
 		$document->applyProfile($profile);
 		
 		ob_start();
 		$document->sendResponse();
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; profile="https://jsonapi.org"'], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Official->value.'; profile="https://jsonapi.org"'], xdebug_get_headers());
 		
 		$profile = new TestProfile();
 		$profile->setOfficialLink('https://jsonapi.org/2');
@@ -132,14 +135,14 @@ public function testSendResponse_ContentTypeHeaderWithProfiles() {
 		ob_start();
 		$document->sendResponse();
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; profile="https://jsonapi.org https://jsonapi.org/2"'], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.ContentTypeEnum::Official->value.'; profile="https://jsonapi.org https://jsonapi.org/2"'], xdebug_get_headers());
 	}
 	
 	/**
 	 * @runInSeparateProcess
 	 */
 	public function testSendResponse_StatusCodeHeader() {
-		$document = new Document();
+		$document = new TestableNonAbstractDocument();
 		
 		ob_start();
 		$document->sendResponse();
@@ -169,7 +172,7 @@ public function testSendResponse_StatusCodeHeader() {
 	 * @runInSeparateProcess
 	 */
 	public function testSendResponse_CustomJson() {
-		$document = new Document();
+		$document = new TestableNonAbstractDocument();
 		$options  = ['json' => '{"foo":42}'];
 		
 		ob_start();
diff --git a/tests/TestableNonAbstractDocument.php b/tests/TestableNonAbstractDocument.php
index 4f1b018..57f5516 100644
--- a/tests/TestableNonAbstractDocument.php
+++ b/tests/TestableNonAbstractDocument.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
 use alsvanzelf\jsonapi\Document;
diff --git a/tests/ValidatorTest.php b/tests/ValidatorTest.php
index 0a373c6..79286a7 100644
--- a/tests/ValidatorTest.php
+++ b/tests/ValidatorTest.php
@@ -1,14 +1,17 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests;
 
-use PHPUnit\Framework\Attributes\DataProvider;
-use PHPUnit\Framework\Attributes\DoesNotPerformAssertions;
-use PHPUnit\Framework\TestCase;
+use alsvanzelf\jsonapi\enums\ObjectContainerEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\objects\ResourceObject;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\DoesNotPerformAssertions;
+use PHPUnit\Framework\TestCase;
 
 class ValidatorTest extends TestCase {
 	#[DoesNotPerformAssertions]
@@ -16,11 +19,11 @@ public function testClaimUsedFields_HappyPath() {
 		$validator = new Validator();
 		
 		$fieldNames      = ['foo'];
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
 		$fieldNames      = ['bar'];
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 	}
 	
@@ -28,12 +31,12 @@ public function testClaimUsedFields_EnforceNamespace() {
 		$validator  = new Validator();
 		$fieldNames = ['foo'];
 		
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
 		$this->expectException(DuplicateException::class);
 		
-		$objectContainer = Validator::OBJECT_CONTAINER_RELATIONSHIPS;
+		$objectContainer = ObjectContainerEnum::Relationships;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 	}
 	
@@ -42,10 +45,10 @@ public function testClaimUsedFields_AllowSameContainer() {
 		$validator  = new Validator();
 		$fieldNames = ['foo'];
 		
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 	}
 	
@@ -54,10 +57,10 @@ public function testClaimUsedFields_OptionForReusingTypeField() {
 		$validator  = new Validator();
 		$fieldNames = ['type'];
 		
-		$objectContainer = Validator::OBJECT_CONTAINER_TYPE;
+		$objectContainer = ObjectContainerEnum::Type;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$options         = ['enforceTypeFieldNamespace' => false];
 		$validator->claimUsedFields($fieldNames, $objectContainer, $options);
 	}
@@ -67,7 +70,7 @@ public function testClearUsedFields_HappyPath() {
 		$validator       = new Validator();
 		
 		$fieldNames      = ['foo'];
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
 		$validator->clearUsedFields($objectContainer);
@@ -77,13 +80,13 @@ public function testClearUsedFields_FreesForAnotherNamespace() {
 		$validator  = new Validator();
 		
 		$fieldNames      = ['foo', 'bar'];
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
 		$thrown = false;
 		try {
 			$fieldNames      = ['bar'];
-			$objectContainer = Validator::OBJECT_CONTAINER_RELATIONSHIPS;
+			$objectContainer = ObjectContainerEnum::Relationships;
 			$validator->claimUsedFields($fieldNames, $objectContainer);
 		}
 		catch (DuplicateException) {
@@ -91,21 +94,21 @@ public function testClearUsedFields_FreesForAnotherNamespace() {
 		}
 		$this->assertTrue($thrown);
 		
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->clearUsedFields($objectContainer);
 		
 		$fieldNames      = ['foo'];
-		$objectContainer = Validator::OBJECT_CONTAINER_ATTRIBUTES;
+		$objectContainer = ObjectContainerEnum::Attributes;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
 		$fieldNames      = ['bar'];
-		$objectContainer = Validator::OBJECT_CONTAINER_RELATIONSHIPS;
+		$objectContainer = ObjectContainerEnum::Relationships;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 		
 		$this->expectException(DuplicateException::class);
 		
 		$fieldNames      = ['foo'];
-		$objectContainer = Validator::OBJECT_CONTAINER_RELATIONSHIPS;
+		$objectContainer = ObjectContainerEnum::Relationships;
 		$validator->claimUsedFields($fieldNames, $objectContainer);
 	}
 	
diff --git a/tests/bootstrap_tests.php b/tests/bootstrap_tests.php
index f695b44..80a03b3 100644
--- a/tests/bootstrap_tests.php
+++ b/tests/bootstrap_tests.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 error_reporting(-1);
 
 require_once __DIR__.'/../vendor/autoload.php';
diff --git a/tests/example_output/ExampleEverywhereExtension.php b/tests/example_output/ExampleEverywhereExtension.php
index 885e26b..17a05b1 100644
--- a/tests/example_output/ExampleEverywhereExtension.php
+++ b/tests/example_output/ExampleEverywhereExtension.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output;
 
 use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
diff --git a/tests/example_output/ExampleTimestampsProfile.php b/tests/example_output/ExampleTimestampsProfile.php
index db6aaa8..f8174ac 100644
--- a/tests/example_output/ExampleTimestampsProfile.php
+++ b/tests/example_output/ExampleTimestampsProfile.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output;
 
 use alsvanzelf\jsonapi\exceptions\InputException;
diff --git a/tests/example_output/ExampleUser.php b/tests/example_output/ExampleUser.php
index 5f342d7..b01edab 100644
--- a/tests/example_output/ExampleUser.php
+++ b/tests/example_output/ExampleUser.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output;
 
 class ExampleUser {
diff --git a/tests/example_output/ExampleVersionExtension.php b/tests/example_output/ExampleVersionExtension.php
index fc80ffb..40987c7 100644
--- a/tests/example_output/ExampleVersionExtension.php
+++ b/tests/example_output/ExampleVersionExtension.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output;
 
 use alsvanzelf\jsonapi\ResourceDocument;
diff --git a/tests/example_output/at_members_everywhere/at_members_everywhere.php b/tests/example_output/at_members_everywhere/at_members_everywhere.php
index 063d068..a6774a0 100644
--- a/tests/example_output/at_members_everywhere/at_members_everywhere.php
+++ b/tests/example_output/at_members_everywhere/at_members_everywhere.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\at_members_everywhere;
 
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\AttributesObject;
 use alsvanzelf\jsonapi\objects\JsonapiObject;
 use alsvanzelf\jsonapi\objects\LinkObject;
@@ -87,7 +90,7 @@ public static function createJsonapiDocument() {
 		$metaObject = new MetaObject();
 		$metaObject->addAtMember('context', '/data/relationships/foo/meta/@context');
 		
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->addAtMember('context', '/data/relationships/foo/@context');
 		$relationshipObject->setResource($resourceObject);
 		$relationshipObject->setLinksObject($linksObject);
@@ -102,7 +105,7 @@ public static function createJsonapiDocument() {
 		$resourceIdentifierObject->addAtMember('context', '/data/relationships/bar/data/@context');
 		$resourceIdentifierObject->setMetaObject($metaObject);
 		
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->addAtMember('context', '/data/relationships/bar/@context');
 		$relationshipObject->setResource($resourceIdentifierObject);
 		
diff --git a/tests/example_output/at_members_in_errors/at_members_in_errors.php b/tests/example_output/at_members_in_errors/at_members_in_errors.php
index 8d97513..e937eb8 100644
--- a/tests/example_output/at_members_in_errors/at_members_in_errors.php
+++ b/tests/example_output/at_members_in_errors/at_members_in_errors.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\at_members_in_errors;
 
 use alsvanzelf\jsonapi\ErrorsDocument;
diff --git a/tests/example_output/collection/collection.php b/tests/example_output/collection/collection.php
index c3c388a..74a2b4b 100644
--- a/tests/example_output/collection/collection.php
+++ b/tests/example_output/collection/collection.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\collection;
 
 use alsvanzelf\jsonapi\CollectionDocument;
diff --git a/tests/example_output/collection_canonical/collection_canonical.php b/tests/example_output/collection_canonical/collection_canonical.php
index 00cb559..58edac7 100644
--- a/tests/example_output/collection_canonical/collection_canonical.php
+++ b/tests/example_output/collection_canonical/collection_canonical.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\collection_canonical;
 
 use alsvanzelf\jsonapi\CollectionDocument;
diff --git a/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.php b/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.php
index dc4db89..3af39a2 100644
--- a/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.php
+++ b/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\cursor_pagination_profile;
 
 use alsvanzelf\jsonapi\CollectionDocument;
diff --git a/tests/example_output/errors_all_options/errors_all_options.json b/tests/example_output/errors_all_options/errors_all_options.json
index 6163dc5..82dd68d 100644
--- a/tests/example_output/errors_all_options/errors_all_options.json
+++ b/tests/example_output/errors_all_options/errors_all_options.json
@@ -64,7 +64,7 @@
 				"message": "please don't throw things",
 				"code": 500,
 				"file": "/errors_all_options.php",
-				"line": 31
+				"line": 33
 			}
 		},
 		{
@@ -74,7 +74,7 @@
 				"message": "something went wrong!",
 				"code": 0,
 				"file": "/errors_all_options.php",
-				"line": 30
+				"line": 32
 			}
 		},
 		{
diff --git a/tests/example_output/errors_all_options/errors_all_options.php b/tests/example_output/errors_all_options/errors_all_options.php
index 5c138ce..c0e563a 100644
--- a/tests/example_output/errors_all_options/errors_all_options.php
+++ b/tests/example_output/errors_all_options/errors_all_options.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\errors_all_options;
 
 use alsvanzelf\jsonapi\ErrorsDocument;
diff --git a/tests/example_output/errors_exception_native/errors_exception_native.json b/tests/example_output/errors_exception_native/errors_exception_native.json
index 3857d56..9a9998c 100644
--- a/tests/example_output/errors_exception_native/errors_exception_native.json
+++ b/tests/example_output/errors_exception_native/errors_exception_native.json
@@ -11,7 +11,7 @@
 				"message": "unknown user",
 				"code": 404,
 				"file": "/errors_exception_native.php",
-				"line": 9
+				"line": 11
 			}
 		}
 	]
diff --git a/tests/example_output/errors_exception_native/errors_exception_native.php b/tests/example_output/errors_exception_native/errors_exception_native.php
index ddac1a3..9cc0e54 100644
--- a/tests/example_output/errors_exception_native/errors_exception_native.php
+++ b/tests/example_output/errors_exception_native/errors_exception_native.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\errors_exception_native;
 
 use alsvanzelf\jsonapi\ErrorsDocument;
diff --git a/tests/example_output/extension/extension.php b/tests/example_output/extension/extension.php
index 4535c63..8870a33 100644
--- a/tests/example_output/extension/extension.php
+++ b/tests/example_output/extension/extension.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\extension;
 
 use alsvanzelf\jsonapi\ResourceDocument;
diff --git a/tests/example_output/extension_members_everywhere/extension_members_everywhere.php b/tests/example_output/extension_members_everywhere/extension_members_everywhere.php
index f7747f2..6790a1f 100644
--- a/tests/example_output/extension_members_everywhere/extension_members_everywhere.php
+++ b/tests/example_output/extension_members_everywhere/extension_members_everywhere.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\extension_members_everywhere;
 
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\AttributesObject;
 use alsvanzelf\jsonapi\objects\JsonapiObject;
 use alsvanzelf\jsonapi\objects\LinkObject;
@@ -92,7 +95,7 @@ public static function createJsonapiDocument() {
 		$metaObject = new MetaObject();
 		$metaObject->addExtensionMember($extension, 'key', '/data/relationships/foo/meta/key');
 		
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->addExtensionMember($extension, 'key', '/data/relationships/foo/key');
 		$relationshipObject->setResource($resourceObject);
 		$relationshipObject->setLinksObject($linksObject);
@@ -107,7 +110,7 @@ public static function createJsonapiDocument() {
 		$resourceIdentifierObject->addExtensionMember($extension, 'key', '/data/relationships/bar/data/key');
 		$resourceIdentifierObject->setMetaObject($metaObject);
 		
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->addExtensionMember($extension, 'key', '/data/relationships/bar/key');
 		$relationshipObject->setResource($resourceIdentifierObject);
 		
diff --git a/tests/example_output/meta_only/meta_only.php b/tests/example_output/meta_only/meta_only.php
index 2bf1ee6..2bf360d 100644
--- a/tests/example_output/meta_only/meta_only.php
+++ b/tests/example_output/meta_only/meta_only.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\meta_only;
 
 use alsvanzelf\jsonapi\MetaDocument;
diff --git a/tests/example_output/null_values/null_values.php b/tests/example_output/null_values/null_values.php
index 7c4b755..488910b 100644
--- a/tests/example_output/null_values/null_values.php
+++ b/tests/example_output/null_values/null_values.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\null_values;
 
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\LinkObject;
 use alsvanzelf\jsonapi\objects\RelationshipObject;
 
@@ -17,8 +20,8 @@ public static function createJsonapiDocument() {
 		$document->addLinkObject('bar', new LinkObject());
 		
 		$document->addRelationship('bar', null);
-		$document->addRelationshipObject('baz', new RelationshipObject(RelationshipObject::TO_ONE));
-		$document->addRelationshipObject('baf', new RelationshipObject(RelationshipObject::TO_MANY));
+		$document->addRelationshipObject('baz', new RelationshipObject(RelationshipTypeEnum::ToOne));
+		$document->addRelationshipObject('baf', new RelationshipObject(RelationshipTypeEnum::ToMany));
 		
 		return $document;
 	}
diff --git a/tests/example_output/profile/profile.php b/tests/example_output/profile/profile.php
index 16deba8..56bdde4 100644
--- a/tests/example_output/profile/profile.php
+++ b/tests/example_output/profile/profile.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\profile;
 
 use alsvanzelf\jsonapi\ResourceDocument;
diff --git a/tests/example_output/relationship_to_many_document/relationship_to_many_document.php b/tests/example_output/relationship_to_many_document/relationship_to_many_document.php
index 5c7b363..d5ea356 100644
--- a/tests/example_output/relationship_to_many_document/relationship_to_many_document.php
+++ b/tests/example_output/relationship_to_many_document/relationship_to_many_document.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\relationship_to_many_document;
 
 use alsvanzelf\jsonapi\CollectionDocument;
diff --git a/tests/example_output/relationship_to_one_document/relationship_to_one_document.php b/tests/example_output/relationship_to_one_document/relationship_to_one_document.php
index 3a19d29..4da6f75 100644
--- a/tests/example_output/relationship_to_one_document/relationship_to_one_document.php
+++ b/tests/example_output/relationship_to_one_document/relationship_to_one_document.php
@@ -1,15 +1,17 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\relationship_to_one_document;
 
-use alsvanzelf\jsonapi\Document;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 
 class relationship_to_one_document {
 	public static function createJsonapiDocument() {
 		$document = new ResourceDocument('author', 12);
 		
-		$document->setSelfLink('/articles/1/relationship/author', $meta=[], $level=Document::LEVEL_ROOT);
+		$document->setSelfLink('/articles/1/relationship/author', $meta=[], $level=DocumentLevelEnum::Root);
 		$document->addLink('related', '/articles/1/author');
 		
 		return $document;
diff --git a/tests/example_output/relationships/relationships.php b/tests/example_output/relationships/relationships.php
index 8426cdd..49f833f 100644
--- a/tests/example_output/relationships/relationships.php
+++ b/tests/example_output/relationships/relationships.php
@@ -1,9 +1,12 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\relationships;
 
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\RelationshipObject;
 use alsvanzelf\jsonapi\objects\ResourceObject;
 
@@ -43,7 +46,7 @@ public static function createJsonapiDocument() {
 		 * to-many relationship, one-by-one
 		 */
 		
-		$relationshipObject = new RelationshipObject($type=RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject($type=RelationshipTypeEnum::ToMany);
 		$relationshipObject->addResource($friend1Resource);
 		$relationshipObject->addResource($friend2Resource);
 		
@@ -63,7 +66,7 @@ public static function createJsonapiDocument() {
 		 * to-many relationship, different types
 		 */
 		
-		$relationshipObject = new RelationshipObject($type=RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject($type=RelationshipTypeEnum::ToMany);
 		$relationshipObject->addResource($ship1Resource);
 		$relationshipObject->addResource($dockResource);
 		
diff --git a/tests/example_output/resource_document_identifier_only/resource_document_identifier_only.php b/tests/example_output/resource_document_identifier_only/resource_document_identifier_only.php
index 493f7cc..3c96058 100644
--- a/tests/example_output/resource_document_identifier_only/resource_document_identifier_only.php
+++ b/tests/example_output/resource_document_identifier_only/resource_document_identifier_only.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\resource_document_identifier_only;
 
 use alsvanzelf\jsonapi\ResourceDocument;
diff --git a/tests/example_output/resource_human_api/resource_human_api.php b/tests/example_output/resource_human_api/resource_human_api.php
index 3d9b622..5505ce4 100644
--- a/tests/example_output/resource_human_api/resource_human_api.php
+++ b/tests/example_output/resource_human_api/resource_human_api.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\resource_human_api;
 
 use alsvanzelf\jsonapi\ResourceDocument;
diff --git a/tests/example_output/resource_links/resource_links.json b/tests/example_output/resource_links/resource_links.json
index 95cfa06..5942883 100644
--- a/tests/example_output/resource_links/resource_links.json
+++ b/tests/example_output/resource_links/resource_links.json
@@ -6,7 +6,7 @@
 		"redirect": {
 			"href": "/login",
 			"meta": {
-				"level": "root"
+				"level": "Root"
 			}
 		}
 	},
@@ -22,13 +22,13 @@
 			"self": {
 				"href": "/user/42",
 				"meta": {
-					"level": "resource"
+					"level": "Resource"
 				}
 			},
 			"partner": {
 				"href": "/user/1",
 				"meta": {
-					"level": "resource"
+					"level": "Resource"
 				}
 			}
 		}
diff --git a/tests/example_output/resource_links/resource_links.php b/tests/example_output/resource_links/resource_links.php
index 7d42d5f..2da4472 100644
--- a/tests/example_output/resource_links/resource_links.php
+++ b/tests/example_output/resource_links/resource_links.php
@@ -1,9 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\resource_links;
 
-use alsvanzelf\jsonapi\Document;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\DocumentLevelEnum;
 use alsvanzelf\jsonapiTests\example_output\ExampleUser;
 
 class resource_links {
@@ -14,13 +16,13 @@ public static function createJsonapiDocument() {
 		
 		$document = ResourceDocument::fromObject($user42, $type='user', $user42->id);
 		
-		$selfResourceMeta = ['level' => Document::LEVEL_RESOURCE];
-		$partnerMeta      = ['level' => Document::LEVEL_RESOURCE];
-		$redirectMeta     = ['level' => Document::LEVEL_ROOT];
+		$selfResourceMeta = ['level' => DocumentLevelEnum::Resource->name];
+		$partnerMeta      = ['level' => DocumentLevelEnum::Resource->name];
+		$redirectMeta     = ['level' => DocumentLevelEnum::Root->name];
 		
 		$document->setSelfLink('/user/42',        $selfResourceMeta);
-		$document->addLink('partner',  '/user/1', $partnerMeta,  $level=Document::LEVEL_RESOURCE);
-		$document->addLink('redirect', '/login',  $redirectMeta, $level=Document::LEVEL_ROOT);
+		$document->addLink('partner',  '/user/1', $partnerMeta,  $level=DocumentLevelEnum::Resource);
+		$document->addLink('redirect', '/login',  $redirectMeta, $level=DocumentLevelEnum::Root);
 		
 		return $document;
 	}
diff --git a/tests/example_output/resource_nested_relations/resource_nested_relations.php b/tests/example_output/resource_nested_relations/resource_nested_relations.php
index 4095c84..5db1d54 100644
--- a/tests/example_output/resource_nested_relations/resource_nested_relations.php
+++ b/tests/example_output/resource_nested_relations/resource_nested_relations.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\resource_nested_relations;
 
 use alsvanzelf\jsonapi\ResourceDocument;
diff --git a/tests/example_output/resource_spec_api/resource_spec_api.php b/tests/example_output/resource_spec_api/resource_spec_api.php
index a3c9b17..62bcc5e 100644
--- a/tests/example_output/resource_spec_api/resource_spec_api.php
+++ b/tests/example_output/resource_spec_api/resource_spec_api.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\resource_spec_api;
 
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\objects\AttributesObject;
 use alsvanzelf\jsonapi\objects\LinksObject;
 use alsvanzelf\jsonapi\objects\MetaObject;
@@ -43,7 +46,7 @@ public static function createJsonapiDocument() {
 		$resource->setType('user');
 		$resource->setAttributesObject($attributes42);
 		
-		$relationship = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationship = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationship->setResource($resource);
 		$relationships = new RelationshipsObject();
 		$relationships->addRelationshipObject('friend', $relationship);
diff --git a/tests/example_output/status_only/status_only.php b/tests/example_output/status_only/status_only.php
index 3546205..209e9cb 100644
--- a/tests/example_output/status_only/status_only.php
+++ b/tests/example_output/status_only/status_only.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\example_output\status_only;
 
 use alsvanzelf\jsonapi\MetaDocument;
diff --git a/tests/extensions/AtomicOperationsDocumentTest.php b/tests/extensions/AtomicOperationsDocumentTest.php
index e1bac4f..d4f5c40 100644
--- a/tests/extensions/AtomicOperationsDocumentTest.php
+++ b/tests/extensions/AtomicOperationsDocumentTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\extensions;
 
 use alsvanzelf\jsonapi\extensions\AtomicOperationsDocument;
diff --git a/tests/extensions/TestExtension.php b/tests/extensions/TestExtension.php
index 9b57f9e..42c72b0 100644
--- a/tests/extensions/TestExtension.php
+++ b/tests/extensions/TestExtension.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\extensions;
 
 use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
diff --git a/tests/helpers/AtMemberManagerTest.php b/tests/helpers/AtMemberManagerTest.php
index b0244aa..ee2fc21 100644
--- a/tests/helpers/AtMemberManagerTest.php
+++ b/tests/helpers/AtMemberManagerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\exceptions\InputException;
diff --git a/tests/helpers/ExtensionMemberManagerTest.php b/tests/helpers/ExtensionMemberManagerTest.php
index 745cd64..16c4933 100644
--- a/tests/helpers/ExtensionMemberManagerTest.php
+++ b/tests/helpers/ExtensionMemberManagerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\exceptions\InputException;
diff --git a/tests/helpers/HttpStatusCodeManagerTest.php b/tests/helpers/HttpStatusCodeManagerTest.php
index bb22b96..60ffc3d 100644
--- a/tests/helpers/HttpStatusCodeManagerTest.php
+++ b/tests/helpers/HttpStatusCodeManagerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\exceptions\InputException;
diff --git a/tests/helpers/LinksManagerTest.php b/tests/helpers/LinksManagerTest.php
index b93ef1f..97c462b 100644
--- a/tests/helpers/LinksManagerTest.php
+++ b/tests/helpers/LinksManagerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\objects\LinkObject;
diff --git a/tests/helpers/RequestParserTest.php b/tests/helpers/RequestParserTest.php
index e3d6311..529696e 100644
--- a/tests/helpers/RequestParserTest.php
+++ b/tests/helpers/RequestParserTest.php
@@ -1,8 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
-use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\enums\ContentTypeEnum;
+use alsvanzelf\jsonapi\enums\SortOrderEnum;
 use alsvanzelf\jsonapi\helpers\RequestParser;
 use alsvanzelf\jsonapiTests\helpers\TestableNonInterfaceRequestInterface;
 use alsvanzelf\jsonapiTests\helpers\TestableNonInterfaceServerRequestInterface;
@@ -26,7 +29,7 @@ public function testFromSuperglobals_HappyPath() {
 		$_SERVER['REQUEST_SCHEME'] = 'https';
 		$_SERVER['HTTP_HOST']      = 'example.org';
 		$_SERVER['REQUEST_URI']    = '/user/42?'.http_build_query($_GET);
-		$_SERVER['CONTENT_TYPE']   = Document::CONTENT_TYPE_OFFICIAL;
+		$_SERVER['CONTENT_TYPE']   = ContentTypeEnum::Official->value;
 		
 		$_POST = [
 			'data' => [
@@ -61,7 +64,7 @@ public function testFromSuperglobals_HappyPath() {
 		
 		$this->assertSame(['ship' => ['wing' => []]], $requestParser->getIncludePaths());
 		$this->assertSame(['name', 'location'], $requestParser->getSparseFieldset('user'));
-		$this->assertSame([['field' => 'name', 'order' => RequestParser::SORT_ASCENDING], ['field' => 'location', 'order' => RequestParser::SORT_DESCENDING]], $requestParser->getSortFields());
+		$this->assertSame([['field' => 'name', 'order' => SortOrderEnum::Ascending], ['field' => 'location', 'order' => SortOrderEnum::Descending]], $requestParser->getSortFields());
 		$this->assertSame(['number' => '2', 'size' => '10'], $requestParser->getPagination());
 		$this->assertSame('42', $requestParser->getFilter());
 		
@@ -80,7 +83,7 @@ public function testFromSuperglobals_WithPhpInputStream() {
 		$_SERVER['REQUEST_SCHEME'] = 'https';
 		$_SERVER['HTTP_HOST']      = 'example.org';
 		$_SERVER['REQUEST_URI']    = '/';
-		$_SERVER['CONTENT_TYPE']   = Document::CONTENT_TYPE_OFFICIAL;
+		$_SERVER['CONTENT_TYPE']   = ContentTypeEnum::Official->value;
 		
 		$_GET  = [];
 		$_POST = [];
@@ -152,7 +155,7 @@ public function testFromPsrRequest_WithRequestInterface() {
 		
 		$this->assertSame(['ship' => ['wing' => []]], $requestParser->getIncludePaths());
 		$this->assertSame(['name', 'location'], $requestParser->getSparseFieldset('user'));
-		$this->assertSame([['field' => 'name', 'order' => RequestParser::SORT_ASCENDING], ['field' => 'location', 'order' => RequestParser::SORT_DESCENDING]], $requestParser->getSortFields());
+		$this->assertSame([['field' => 'name', 'order' => SortOrderEnum::Ascending], ['field' => 'location', 'order' => SortOrderEnum::Descending]], $requestParser->getSortFields());
 		$this->assertSame(['number' => '2', 'size' => '10'], $requestParser->getPagination());
 		$this->assertSame('42', $requestParser->getFilter());
 		
@@ -190,7 +193,7 @@ public function testFromPsrRequest_WithServerRequestInterface() {
 		
 		$this->assertSame('https://example.org/user/42?'.http_build_query($queryParameters), $requestParser->getSelfLink());
 		$this->assertTrue($requestParser->hasSortFields());
-		$this->assertSame([['field' => 'name', 'order' => RequestParser::SORT_ASCENDING], ['field' => 'location', 'order' => RequestParser::SORT_DESCENDING]], $requestParser->getSortFields());
+		$this->assertSame([['field' => 'name', 'order' => SortOrderEnum::Ascending], ['field' => 'location', 'order' => SortOrderEnum::Descending]], $requestParser->getSortFields());
 	}
 	
 	public function testGetSelfLink() {
@@ -292,15 +295,15 @@ public function testHasSortFields() {
 	public function testGetSortFields_Reformatted() {
 		$queryParameters = ['sort' => 'foo'];
 		$requestParser = new RequestParser($selfLink='', $queryParameters);
-		$this->assertSame([['field' => 'foo', 'order' => RequestParser::SORT_ASCENDING]], $requestParser->getSortFields());
+		$this->assertSame([['field' => 'foo', 'order' => SortOrderEnum::Ascending]], $requestParser->getSortFields());
 		
 		$queryParameters = ['sort' => '-bar'];
 		$requestParser = new RequestParser($selfLink='', $queryParameters);
-		$this->assertSame([['field' => 'bar', 'order' => RequestParser::SORT_DESCENDING]], $requestParser->getSortFields());
+		$this->assertSame([['field' => 'bar', 'order' => SortOrderEnum::Descending]], $requestParser->getSortFields());
 		
 		$queryParameters = ['sort' => 'foo,-bar'];
 		$requestParser = new RequestParser($selfLink='', $queryParameters);
-		$this->assertSame([['field' => 'foo', 'order' => RequestParser::SORT_ASCENDING], ['field' => 'bar', 'order' => RequestParser::SORT_DESCENDING]], $requestParser->getSortFields());
+		$this->assertSame([['field' => 'foo', 'order' => SortOrderEnum::Ascending], ['field' => 'bar', 'order' => SortOrderEnum::Descending]], $requestParser->getSortFields());
 	}
 	
 	public function testGetSortFields_Raw() {
diff --git a/tests/helpers/TestableNonInterfaceRequestInterface.php b/tests/helpers/TestableNonInterfaceRequestInterface.php
index e2d26da..1918c52 100644
--- a/tests/helpers/TestableNonInterfaceRequestInterface.php
+++ b/tests/helpers/TestableNonInterfaceRequestInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapiTests\helpers\TestableNonInterfaceUriInterface;
diff --git a/tests/helpers/TestableNonInterfaceServerRequestInterface.php b/tests/helpers/TestableNonInterfaceServerRequestInterface.php
index 5794c86..c732b1b 100644
--- a/tests/helpers/TestableNonInterfaceServerRequestInterface.php
+++ b/tests/helpers/TestableNonInterfaceServerRequestInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapiTests\helpers\TestableNonInterfaceRequestInterface;
diff --git a/tests/helpers/TestableNonInterfaceStreamInterface.php b/tests/helpers/TestableNonInterfaceStreamInterface.php
index 79f22ca..47e4b5c 100644
--- a/tests/helpers/TestableNonInterfaceStreamInterface.php
+++ b/tests/helpers/TestableNonInterfaceStreamInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use Psr\Http\Message\StreamInterface;
diff --git a/tests/helpers/TestableNonInterfaceUriInterface.php b/tests/helpers/TestableNonInterfaceUriInterface.php
index aad6bd1..97d8359 100644
--- a/tests/helpers/TestableNonInterfaceUriInterface.php
+++ b/tests/helpers/TestableNonInterfaceUriInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use Psr\Http\Message\UriInterface;
diff --git a/tests/helpers/TestableNonTraitAtMemberManager.php b/tests/helpers/TestableNonTraitAtMemberManager.php
index d1c852f..1cea283 100644
--- a/tests/helpers/TestableNonTraitAtMemberManager.php
+++ b/tests/helpers/TestableNonTraitAtMemberManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
diff --git a/tests/helpers/TestableNonTraitExtensionMemberManager.php b/tests/helpers/TestableNonTraitExtensionMemberManager.php
index 516f103..6e699c5 100644
--- a/tests/helpers/TestableNonTraitExtensionMemberManager.php
+++ b/tests/helpers/TestableNonTraitExtensionMemberManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
diff --git a/tests/helpers/TestableNonTraitHttpStatusCodeManager.php b/tests/helpers/TestableNonTraitHttpStatusCodeManager.php
index 6fff802..ade421b 100644
--- a/tests/helpers/TestableNonTraitHttpStatusCodeManager.php
+++ b/tests/helpers/TestableNonTraitHttpStatusCodeManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\helpers\HttpStatusCodeManager;
diff --git a/tests/helpers/TestableNonTraitLinksManager.php b/tests/helpers/TestableNonTraitLinksManager.php
index 478fe62..3453fb9 100644
--- a/tests/helpers/TestableNonTraitLinksManager.php
+++ b/tests/helpers/TestableNonTraitLinksManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\helpers;
 
 use alsvanzelf\jsonapi\helpers\LinksManager;
diff --git a/tests/objects/AttributesObjectTest.php b/tests/objects/AttributesObjectTest.php
index 047e3d1..68f49da 100644
--- a/tests/objects/AttributesObjectTest.php
+++ b/tests/objects/AttributesObjectTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\exceptions\InputException;
diff --git a/tests/objects/ErrorObjectTest.php b/tests/objects/ErrorObjectTest.php
index a794dca..3136520 100644
--- a/tests/objects/ErrorObjectTest.php
+++ b/tests/objects/ErrorObjectTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\exceptions\InputException;
diff --git a/tests/objects/JsonapiObjectTest.php b/tests/objects/JsonapiObjectTest.php
index 882c708..a2c3c4e 100644
--- a/tests/objects/JsonapiObjectTest.php
+++ b/tests/objects/JsonapiObjectTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\objects\JsonapiObject;
diff --git a/tests/objects/LinkObjectTest.php b/tests/objects/LinkObjectTest.php
index fd356c9..c37d137 100644
--- a/tests/objects/LinkObjectTest.php
+++ b/tests/objects/LinkObjectTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\objects\LinkObject;
diff --git a/tests/objects/LinksObjectTest.php b/tests/objects/LinksObjectTest.php
index 4f1ee7e..c65643d 100644
--- a/tests/objects/LinksObjectTest.php
+++ b/tests/objects/LinksObjectTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
diff --git a/tests/objects/MetaObjectTest.php b/tests/objects/MetaObjectTest.php
index fc4cb6a..186f97b 100644
--- a/tests/objects/MetaObjectTest.php
+++ b/tests/objects/MetaObjectTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\objects\MetaObject;
diff --git a/tests/objects/RelationshipObjectTest.php b/tests/objects/RelationshipObjectTest.php
index a09dec1..b941fea 100644
--- a/tests/objects/RelationshipObjectTest.php
+++ b/tests/objects/RelationshipObjectTest.php
@@ -1,9 +1,12 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\objects\LinkObject;
 use alsvanzelf\jsonapi\objects\RelationshipObject;
@@ -14,25 +17,19 @@
 
 class RelationshipObjectTest extends TestCase {
 	public function testConstructor_ToOne() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->setResource(new ResourceObject('user', 42));
 		
 		$this->validateToOneRelationshipArray($relationshipObject->toArray());
 	}
 	
 	public function testConstructor_ToMany() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToMany);
 		$relationshipObject->addResource(new ResourceObject('user', 42));
 		
 		$this->validateToManyRelationshipArray($relationshipObject->toArray());
 	}
 	
-	public function testConstructor_UnknownType() {
-		$this->expectException(InputException::class);
-		
-		$relationshipObject = new RelationshipObject('foo');
-	}
-	
 	public function testFromAnything_WithResourceObject() {
 		$resourceObject = new ResourceObject('user', 42);
 		$resourceObject->addMeta('foo', 'bar');
@@ -80,7 +77,7 @@ public function testFromAnything_WithUnknownType() {
 	
 	public function testFromResource_ToMany() {
 		$resourceObject = new ResourceObject('user', 42);
-		$type           = RelationshipObject::TO_MANY;
+		$type           = RelationshipTypeEnum::ToMany;
 		
 		$relationshipObject = RelationshipObject::fromResource($resourceObject, $links=[], $meta=[], $type);
 		
@@ -137,7 +134,7 @@ public function testFromCollectionDocument_WithMeta() {
 	}
 	
 	public function testSetSelfLink_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->setSelfLink('https://jsonapi.org');
 		
 		$array = $relationshipObject->toArray();
@@ -148,7 +145,7 @@ public function testSetSelfLink_HappyPath() {
 	}
 	
 	public function testSetRelatedLink_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->setRelatedLink('https://jsonapi.org');
 		
 		$array = $relationshipObject->toArray();
@@ -159,7 +156,7 @@ public function testSetRelatedLink_HappyPath() {
 	}
 	
 	public function testSetPaginationLinks_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToMany);
 		$baseUrl            = 'https://jsonapi.org/?page=';
 		
 		$relationshipObject->setPaginationLinks($baseUrl.'prev', $baseUrl.'next', $baseUrl.'first', $baseUrl.'last');
@@ -179,7 +176,7 @@ public function testSetPaginationLinks_HappyPath() {
 	}
 	
 	public function testSetPaginationLinks_BlockedOnToOne() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		
 		$this->expectException(InputException::class);
 		
@@ -187,7 +184,7 @@ public function testSetPaginationLinks_BlockedOnToOne() {
 	}
 	
 	public function testAddMeta_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		
 		$this->assertTrue($relationshipObject->isEmpty());
 		
@@ -205,7 +202,7 @@ public function testAddMeta_HappyPath() {
 	public function testHasResource_ToMany() {
 		$resourceObject = new ResourceObject('user', 42);
 		
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToMany);
 		$relationshipObject->addResource($resourceObject);
 		
 		$this->assertTrue($relationshipObject->hasResource($resourceObject));
@@ -214,7 +211,7 @@ public function testHasResource_ToMany() {
 	}
 	
 	public function testGetContainedResources_SkipsResourceIdentifierObjects() {
-		$relationshipObject           = new RelationshipObject(RelationshipObject::TO_MANY);
+		$relationshipObject           = new RelationshipObject(RelationshipTypeEnum::ToMany);
 		$resourceIdentifierObject     = new ResourceIdentifierObject('user', 24);
 		$resourceObjectIdentifierOnly = new ResourceObject('user', 42);
 		$resourceObjectWithAttributes = new ResourceObject('user', 42);
@@ -236,14 +233,14 @@ public function testGetContainedResources_SkipsResourceIdentifierObjects() {
 	}
 	
 	public function testSetResource_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->setResource(new ResourceObject('user', 42));
 		
 		$this->validateToOneRelationshipArray($relationshipObject->toArray());
 	}
 	
 	public function testSetResource_RequiresToOneType() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToMany);
 		
 		$this->expectException(InputException::class);
 		
@@ -251,14 +248,14 @@ public function testSetResource_RequiresToOneType() {
 	}
 	
 	public function testAddResource_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToMany);
 		$relationshipObject->addResource(new ResourceObject('user', 42));
 		
 		$this->validateToManyRelationshipArray($relationshipObject->toArray());
 	}
 	
 	public function testAddResource_RequiresToOneType() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		
 		$this->expectException(InputException::class);
 		
@@ -266,7 +263,7 @@ public function testAddResource_RequiresToOneType() {
 	}
 	
 	public function testAddLinkObject_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		
 		$this->assertTrue($relationshipObject->isEmpty());
 		
@@ -284,7 +281,7 @@ public function testAddLinkObject_HappyPath() {
 	}
 	
 	public function testToArray_EmptyResource() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		
 		$array = $relationshipObject->toArray();
 		
@@ -293,7 +290,7 @@ public function testToArray_EmptyResource() {
 	}
 	
 	public function testToArray_EmptyResources() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_MANY);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToMany);
 		
 		$array = $relationshipObject->toArray();
 		
@@ -302,7 +299,7 @@ public function testToArray_EmptyResources() {
 	}
 	
 	public function testIsEmpty_WithAtMembers() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		
 		$this->assertTrue($relationshipObject->isEmpty());
 		
@@ -315,7 +312,7 @@ public function testIsEmpty_WithAtMembers() {
 	 * @group Extensions
 	 */
 	public function testIsEmpty_WithExtensionMembers() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		
 		$this->assertTrue($relationshipObject->isEmpty());
 		
diff --git a/tests/objects/RelationshipsObjectTest.php b/tests/objects/RelationshipsObjectTest.php
index a1eb510..9c98b61 100644
--- a/tests/objects/RelationshipsObjectTest.php
+++ b/tests/objects/RelationshipsObjectTest.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\objects\RelationshipObject;
@@ -94,7 +97,7 @@ public function testAddRelationshipObject_MultipleReusingKeys() {
 	}
 	
 	public function testToArray_EmptyRelationship() {
-		$relationshipObject  = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject  = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipsObject = new RelationshipsObject();
 		
 		$relationshipsObject->addRelationshipObject($key='foo', $relationshipObject);
diff --git a/tests/objects/ResourceIdentifierObjectTest.php b/tests/objects/ResourceIdentifierObjectTest.php
index b7eb1ea..5724491 100644
--- a/tests/objects/ResourceIdentifierObjectTest.php
+++ b/tests/objects/ResourceIdentifierObjectTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\exceptions\Exception;
diff --git a/tests/objects/ResourceObjectTest.php b/tests/objects/ResourceObjectTest.php
index f2873d6..693c859 100644
--- a/tests/objects/ResourceObjectTest.php
+++ b/tests/objects/ResourceObjectTest.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\objects;
 
+use alsvanzelf\jsonapi\enums\RelationshipTypeEnum;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\objects\AttributesObject;
 use alsvanzelf\jsonapi\objects\LinkObject;
@@ -127,7 +130,7 @@ public function testHasIdentifierPropertiesOnly_No() {
 	}
 	
 	public function testAddRelationshipObject_HappyPath() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->setResource(new ResourceObject('user', 42));
 		
 		$resourceObject = new ResourceObject('user', 24);
@@ -151,7 +154,7 @@ public function testAddRelationshipObject_HappyPath() {
 	}
 	
 	public function testAddRelationshipObject_BlockDrosteEffect() {
-		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject = new RelationshipObject(RelationshipTypeEnum::ToOne);
 		$relationshipObject->setResource(new ResourceObject('user', 42));
 		
 		$resourceObject = new ResourceObject('user', 42);
diff --git a/tests/profiles/CursorPaginationProfileTest.php b/tests/profiles/CursorPaginationProfileTest.php
index 41fa3ef..5b9a8c2 100644
--- a/tests/profiles/CursorPaginationProfileTest.php
+++ b/tests/profiles/CursorPaginationProfileTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\profiles;
 
 use alsvanzelf\jsonapi\CollectionDocument;
diff --git a/tests/profiles/TestProfile.php b/tests/profiles/TestProfile.php
index 331e1a8..0e7453b 100644
--- a/tests/profiles/TestProfile.php
+++ b/tests/profiles/TestProfile.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace alsvanzelf\jsonapiTests\profiles;
 
 use alsvanzelf\jsonapi\interfaces\ProfileInterface;