diff --git a/tests/AbstractTestCase.php b/tests/AbstractTestCase.php
index 0310d9e..44e2ab5 100644
--- a/tests/AbstractTestCase.php
+++ b/tests/AbstractTestCase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Api/RequestInterfaceTest.php b/tests/Api/RequestInterfaceTest.php
index 3d40230..336d72c 100644
--- a/tests/Api/RequestInterfaceTest.php
+++ b/tests/Api/RequestInterfaceTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Model/TestMedia.php b/tests/Fixtures/Model/TestMedia.php
index 645f076..5da87b0 100644
--- a/tests/Fixtures/Model/TestMedia.php
+++ b/tests/Fixtures/Model/TestMedia.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Provider/TestApiProvider.php b/tests/Fixtures/Provider/TestApiProvider.php
index fdf679d..aacf2ed 100644
--- a/tests/Fixtures/Provider/TestApiProvider.php
+++ b/tests/Fixtures/Provider/TestApiProvider.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Provider/TestProvider.php b/tests/Fixtures/Provider/TestProvider.php
index 5040891..7aec35f 100644
--- a/tests/Fixtures/Provider/TestProvider.php
+++ b/tests/Fixtures/Provider/TestProvider.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Response/TestMediaResponse.php b/tests/Fixtures/Response/TestMediaResponse.php
index 62a8be5..260e459 100644
--- a/tests/Fixtures/Response/TestMediaResponse.php
+++ b/tests/Fixtures/Response/TestMediaResponse.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Response/TestResponse.php b/tests/Fixtures/Response/TestResponse.php
index 0ad15a3..7f38733 100644
--- a/tests/Fixtures/Response/TestResponse.php
+++ b/tests/Fixtures/Response/TestResponse.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Traits/Integers/TestIntegerTotalResultsTrait.php b/tests/Fixtures/Traits/Integers/TestIntegerTotalResultsTrait.php
index b65db42..0e2b5f5 100644
--- a/tests/Fixtures/Traits/Integers/TestIntegerTotalResultsTrait.php
+++ b/tests/Fixtures/Traits/Integers/TestIntegerTotalResultsTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Traits/Strings/TestStringNextPageTrait.php b/tests/Fixtures/Traits/Strings/TestStringNextPageTrait.php
index 58e239e..9bab726 100644
--- a/tests/Fixtures/Traits/Strings/TestStringNextPageTrait.php
+++ b/tests/Fixtures/Traits/Strings/TestStringNextPageTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Fixtures/Traits/Strings/TestStringPrevPageTrait.php b/tests/Fixtures/Traits/Strings/TestStringPrevPageTrait.php
index 97bee38..a898888 100644
--- a/tests/Fixtures/Traits/Strings/TestStringPrevPageTrait.php
+++ b/tests/Fixtures/Traits/Strings/TestStringPrevPageTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/AbstractMediaTest.php b/tests/Model/AbstractMediaTest.php
index 166d458..066e11a 100644
--- a/tests/Model/AbstractMediaTest.php
+++ b/tests/Model/AbstractMediaTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/CollectionTest.php b/tests/Model/CollectionTest.php
index 31bc976..d3b3a9f 100644
--- a/tests/Model/CollectionTest.php
+++ b/tests/Model/CollectionTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/PhotoTest.php b/tests/Model/PhotoTest.php
index 6d738c4..4d2f669 100644
--- a/tests/Model/PhotoTest.php
+++ b/tests/Model/PhotoTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/SourceTest.php b/tests/Model/SourceTest.php
index bffe262..8350fa2 100644
--- a/tests/Model/SourceTest.php
+++ b/tests/Model/SourceTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/UserTest.php b/tests/Model/UserTest.php
index 16716af..9355a7b 100644
--- a/tests/Model/UserTest.php
+++ b/tests/Model/UserTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/VideoFileTest.php b/tests/Model/VideoFileTest.php
index c78a2a9..9b830bc 100644
--- a/tests/Model/VideoFileTest.php
+++ b/tests/Model/VideoFileTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/VideoPictureTest.php b/tests/Model/VideoPictureTest.php
index b8ddc20..0aa8c73 100644
--- a/tests/Model/VideoPictureTest.php
+++ b/tests/Model/VideoPictureTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Model/VideoTest.php b/tests/Model/VideoTest.php
index 8c15bbe..01bc48a 100644
--- a/tests/Model/VideoTest.php
+++ b/tests/Model/VideoTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Provider/AbstractProviderTest.php b/tests/Provider/AbstractProviderTest.php
index b628326..6470784 100644
--- a/tests/Provider/AbstractProviderTest.php
+++ b/tests/Provider/AbstractProviderTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Provider/ApiProviderTest.php b/tests/Provider/ApiProviderTest.php
index d062b6f..19f187d 100644
--- a/tests/Provider/ApiProviderTest.php
+++ b/tests/Provider/ApiProviderTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/CollectionRequestTest.php b/tests/Request/CollectionRequestTest.php
index fe038c5..81f6a2e 100644
--- a/tests/Request/CollectionRequestTest.php
+++ b/tests/Request/CollectionRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/CollectionsRequestTest.php b/tests/Request/CollectionsRequestTest.php
index 538d631..f697a65 100644
--- a/tests/Request/CollectionsRequestTest.php
+++ b/tests/Request/CollectionsRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/CuratedPhotosRequestTest.php b/tests/Request/CuratedPhotosRequestTest.php
index 54f268c..e12efa1 100644
--- a/tests/Request/CuratedPhotosRequestTest.php
+++ b/tests/Request/CuratedPhotosRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/GetPhotoRequestTest.php b/tests/Request/GetPhotoRequestTest.php
index cae4a8a..879e88c 100644
--- a/tests/Request/GetPhotoRequestTest.php
+++ b/tests/Request/GetPhotoRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/GetVideoRequestTest.php b/tests/Request/GetVideoRequestTest.php
index 03478ff..c55f966 100644
--- a/tests/Request/GetVideoRequestTest.php
+++ b/tests/Request/GetVideoRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/PopularVideosRequestTest.php b/tests/Request/PopularVideosRequestTest.php
index 150f1d7..bba6fc4 100644
--- a/tests/Request/PopularVideosRequestTest.php
+++ b/tests/Request/PopularVideosRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/SearchPhotosRequestTest.php b/tests/Request/SearchPhotosRequestTest.php
index 4829257..ceb0a8b 100644
--- a/tests/Request/SearchPhotosRequestTest.php
+++ b/tests/Request/SearchPhotosRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Request/SearchVideosRequestTest.php b/tests/Request/SearchVideosRequestTest.php
index 326a530..79f4f21 100644
--- a/tests/Request/SearchVideosRequestTest.php
+++ b/tests/Request/SearchVideosRequestTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/AbstractMediaResponseTest.php b/tests/Response/AbstractMediaResponseTest.php
index 6ee13a0..af4a408 100644
--- a/tests/Response/AbstractMediaResponseTest.php
+++ b/tests/Response/AbstractMediaResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/AbstractResponseTest.php b/tests/Response/AbstractResponseTest.php
index c7f6276..fc85fbe 100644
--- a/tests/Response/AbstractResponseTest.php
+++ b/tests/Response/AbstractResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/CollectionResponseTest.php b/tests/Response/CollectionResponseTest.php
index 8ded32c..55dd6b0 100644
--- a/tests/Response/CollectionResponseTest.php
+++ b/tests/Response/CollectionResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/CollectionsResponseTest.php b/tests/Response/CollectionsResponseTest.php
index d9c7bd4..5cc1b79 100644
--- a/tests/Response/CollectionsResponseTest.php
+++ b/tests/Response/CollectionsResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/PhotoResponseTest.php b/tests/Response/PhotoResponseTest.php
index 4757ba4..06dfe86 100644
--- a/tests/Response/PhotoResponseTest.php
+++ b/tests/Response/PhotoResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/PhotosResponseTest.php b/tests/Response/PhotosResponseTest.php
index 55c29f1..1dd65f5 100644
--- a/tests/Response/PhotosResponseTest.php
+++ b/tests/Response/PhotosResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/VideoResponseTest.php b/tests/Response/VideoResponseTest.php
index 94b9d8b..7cebc3f 100644
--- a/tests/Response/VideoResponseTest.php
+++ b/tests/Response/VideoResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Response/VideosResponseTest.php b/tests/Response/VideosResponseTest.php
index 65b1a8a..06b29eb 100644
--- a/tests/Response/VideosResponseTest.php
+++ b/tests/Response/VideosResponseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Serializer/RequestSerializerTest.php b/tests/Serializer/RequestSerializerTest.php
index ef2862c..388c2f6 100644
--- a/tests/Serializer/RequestSerializerTest.php
+++ b/tests/Serializer/RequestSerializerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Serializer/ResponseDeserializerTest.php b/tests/Serializer/ResponseDeserializerTest.php
index 0dfe14e..fa689ce 100644
--- a/tests/Serializer/ResponseDeserializerTest.php
+++ b/tests/Serializer/ResponseDeserializerTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Traits/Integers/IntegerTotalResultsTraitTest.php b/tests/Traits/Integers/IntegerTotalResultsTraitTest.php
index 21e0a06..477ea10 100644
--- a/tests/Traits/Integers/IntegerTotalResultsTraitTest.php
+++ b/tests/Traits/Integers/IntegerTotalResultsTraitTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Traits/Strings/StringNextPageTraitTest.php b/tests/Traits/Strings/StringNextPageTraitTest.php
index cd6d665..68d6851 100644
--- a/tests/Traits/Strings/StringNextPageTraitTest.php
+++ b/tests/Traits/Strings/StringNextPageTraitTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/Traits/Strings/StringPrevPageTraitTest.php b/tests/Traits/Strings/StringPrevPageTraitTest.php
index 594bd48..fd506fe 100644
--- a/tests/Traits/Strings/StringPrevPageTraitTest.php
+++ b/tests/Traits/Strings/StringPrevPageTraitTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/autoload.php.dist b/tests/autoload.php.dist
index e0ba95a..04694b6 100644
--- a/tests/autoload.php.dist
+++ b/tests/autoload.php.dist
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index e8ecfc2..451672c 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types = 1);
+
 /*
  * This file is part of the pexels-library package.
  *