diff --git a/CHANGELOG.md b/CHANGELOG.md
index 395cbd76..c0797555 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 ## 1.7.4 (not yet released)
 * CHANGED: Saving markdown pastes uses `.md` extension instead of `.txt` (#1293)
+* CHANGED: Enable strict type checking in PHP (#1350)
 * FIXED: Reset password input field on creation of new paste (#1194)
 * FIXED: Allow database schema upgrade to skip versions (#1343)
 
diff --git a/bin/administration b/bin/administration
index 0a9a52e7..17b2e0f5 100755
--- a/bin/administration
+++ b/bin/administration
@@ -1,5 +1,5 @@
 #!/usr/bin/env php
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/bin/configuration-test-generator b/bin/configuration-test-generator
index 3a8d0f61..d81c3302 100755
--- a/bin/configuration-test-generator
+++ b/bin/configuration-test-generator
@@ -1,5 +1,5 @@
 #!/usr/bin/env php
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
@@ -413,7 +413,7 @@ class ConfigurationTestGenerator
     private function _getHeader()
     {
         return <<<'EOT'
-<?php
+<?php declare(strict_types=1);
 /**
  * DO NOT EDIT: This file is generated automatically using configGenerator.php
  */
diff --git a/bin/icon-test b/bin/icon-test
index 25c01bc1..abf9f789 100755
--- a/bin/icon-test
+++ b/bin/icon-test
@@ -1,5 +1,5 @@
 #!/usr/bin/env php
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/bin/migrate b/bin/migrate
index a7e42f0d..51aa8f75 100755
--- a/bin/migrate
+++ b/bin/migrate
@@ -1,5 +1,5 @@
 #!/usr/bin/env php
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/index.php b/index.php
index ba3b13a1..154eb6ee 100644
--- a/index.php
+++ b/index.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Configuration.php b/lib/Configuration.php
index 05203f04..59722d15 100644
--- a/lib/Configuration.php
+++ b/lib/Configuration.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Controller.php b/lib/Controller.php
index ab0007b4..d518fcc5 100644
--- a/lib/Controller.php
+++ b/lib/Controller.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Data/AbstractData.php b/lib/Data/AbstractData.php
index c565108f..8e8c8bd5 100644
--- a/lib/Data/AbstractData.php
+++ b/lib/Data/AbstractData.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
@@ -181,7 +181,7 @@ abstract class AbstractData
     protected function getOpenSlot(array &$comments, $postdate)
     {
         if (array_key_exists($postdate, $comments)) {
-            $parts = explode('.', $postdate, 2);
+            $parts = explode('.', (string) $postdate, 2);
             if (!array_key_exists(1, $parts)) {
                 $parts[1] = 0;
             }
diff --git a/lib/Data/Database.php b/lib/Data/Database.php
index 9acbcbe3..addc6c44 100644
--- a/lib/Data/Database.php
+++ b/lib/Data/Database.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Data/Filesystem.php b/lib/Data/Filesystem.php
index 4611dab7..506ab4ee 100644
--- a/lib/Data/Filesystem.php
+++ b/lib/Data/Filesystem.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Data/GoogleCloudStorage.php b/lib/Data/GoogleCloudStorage.php
index ab8c5323..8fa19861 100644
--- a/lib/Data/GoogleCloudStorage.php
+++ b/lib/Data/GoogleCloudStorage.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Data/S3Storage.php b/lib/Data/S3Storage.php
index 9ae598d5..30c705dd 100644
--- a/lib/Data/S3Storage.php
+++ b/lib/Data/S3Storage.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Filter.php b/lib/Filter.php
index d4e79887..27ad05da 100644
--- a/lib/Filter.php
+++ b/lib/Filter.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/FormatV2.php b/lib/FormatV2.php
index a359e267..070c820a 100644
--- a/lib/FormatV2.php
+++ b/lib/FormatV2.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/I18n.php b/lib/I18n.php
index 4fb64bd3..3d1e8029 100644
--- a/lib/I18n.php
+++ b/lib/I18n.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Json.php b/lib/Json.php
index d9c4812b..9985f16e 100644
--- a/lib/Json.php
+++ b/lib/Json.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Model.php b/lib/Model.php
index 4536642a..7c7fa81e 100644
--- a/lib/Model.php
+++ b/lib/Model.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Model/AbstractModel.php b/lib/Model/AbstractModel.php
index 057ae588..fb43400e 100644
--- a/lib/Model/AbstractModel.php
+++ b/lib/Model/AbstractModel.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Model/Comment.php b/lib/Model/Comment.php
index 3a44fae7..5250eb83 100644
--- a/lib/Model/Comment.php
+++ b/lib/Model/Comment.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Model/Paste.php b/lib/Model/Paste.php
index fbb762bc..76dc4b4c 100644
--- a/lib/Model/Paste.php
+++ b/lib/Model/Paste.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Persistence/AbstractPersistence.php b/lib/Persistence/AbstractPersistence.php
index badb73b0..d5d7db6f 100644
--- a/lib/Persistence/AbstractPersistence.php
+++ b/lib/Persistence/AbstractPersistence.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Persistence/PurgeLimiter.php b/lib/Persistence/PurgeLimiter.php
index 23e38e3a..1e87af33 100644
--- a/lib/Persistence/PurgeLimiter.php
+++ b/lib/Persistence/PurgeLimiter.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Persistence/ServerSalt.php b/lib/Persistence/ServerSalt.php
index 91df31a8..4c984b75 100644
--- a/lib/Persistence/ServerSalt.php
+++ b/lib/Persistence/ServerSalt.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Persistence/TrafficLimiter.php b/lib/Persistence/TrafficLimiter.php
index 94caf9bb..e0d3a304 100644
--- a/lib/Persistence/TrafficLimiter.php
+++ b/lib/Persistence/TrafficLimiter.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 /**
  * PrivateBin
diff --git a/lib/Request.php b/lib/Request.php
index 8a2d585c..336b69f1 100644
--- a/lib/Request.php
+++ b/lib/Request.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/View.php b/lib/View.php
index 212894e7..395ac86d 100644
--- a/lib/View.php
+++ b/lib/View.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/lib/Vizhash16x16.php b/lib/Vizhash16x16.php
index 282aa689..729f1f0d 100644
--- a/lib/Vizhash16x16.php
+++ b/lib/Vizhash16x16.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * VizHash_GD
  *
diff --git a/lib/YourlsProxy.php b/lib/YourlsProxy.php
index 795f10cd..f616832d 100644
--- a/lib/YourlsProxy.php
+++ b/lib/YourlsProxy.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 /**
  * PrivateBin
  *
diff --git a/tst/Bootstrap.php b/tst/Bootstrap.php
index 8ea249d2..fba1be14 100644
--- a/tst/Bootstrap.php
+++ b/tst/Bootstrap.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use Google\Cloud\Core\Exception\BadRequestException;
 use Google\Cloud\Core\Exception\NotFoundException;
diff --git a/tst/ConfigurationTest.php b/tst/ConfigurationTest.php
index 687fc46a..27a31f73 100644
--- a/tst/ConfigurationTest.php
+++ b/tst/ConfigurationTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Configuration;
diff --git a/tst/ControllerTest.php b/tst/ControllerTest.php
index b1a5774f..98eb6458 100644
--- a/tst/ControllerTest.php
+++ b/tst/ControllerTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Controller;
diff --git a/tst/ControllerWithDbTest.php b/tst/ControllerWithDbTest.php
index d8ec1114..43ae641d 100644
--- a/tst/ControllerWithDbTest.php
+++ b/tst/ControllerWithDbTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PrivateBin\Data\Database;
 use PrivateBin\Persistence\ServerSalt;
diff --git a/tst/ControllerWithGcsTest.php b/tst/ControllerWithGcsTest.php
index 3e1b828d..e86d119c 100644
--- a/tst/ControllerWithGcsTest.php
+++ b/tst/ControllerWithGcsTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use Google\Auth\HttpHandler\HttpHandlerFactory;
 use GuzzleHttp\Client;
diff --git a/tst/Data/DatabaseTest.php b/tst/Data/DatabaseTest.php
index c903e715..d1003c55 100644
--- a/tst/Data/DatabaseTest.php
+++ b/tst/Data/DatabaseTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Controller;
diff --git a/tst/Data/FilesystemTest.php b/tst/Data/FilesystemTest.php
index 67366be2..76dec363 100644
--- a/tst/Data/FilesystemTest.php
+++ b/tst/Data/FilesystemTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Data\Filesystem;
diff --git a/tst/Data/GoogleCloudStorageTest.php b/tst/Data/GoogleCloudStorageTest.php
index b7cfe349..6ada0c02 100644
--- a/tst/Data/GoogleCloudStorageTest.php
+++ b/tst/Data/GoogleCloudStorageTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use Google\Auth\HttpHandler\HttpHandlerFactory;
 use GuzzleHttp\Client;
diff --git a/tst/FilterTest.php b/tst/FilterTest.php
index 39c4f5df..63361367 100644
--- a/tst/FilterTest.php
+++ b/tst/FilterTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Filter;
diff --git a/tst/FormatV2Test.php b/tst/FormatV2Test.php
index 8b8b6b82..cf6cf565 100644
--- a/tst/FormatV2Test.php
+++ b/tst/FormatV2Test.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\FormatV2;
diff --git a/tst/I18nTest.php b/tst/I18nTest.php
index e606a8b3..14292400 100644
--- a/tst/I18nTest.php
+++ b/tst/I18nTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\I18n;
@@ -198,7 +198,7 @@ class I18nTest extends TestCase
         $languageCount = 0;
         foreach ($languageIterator as $file) {
             ++$languageCount;
-            $this->assertTrue(copy($file, $path . DIRECTORY_SEPARATOR . $file->getBasename()));
+            $this->assertTrue(copy($file->getPathname(), $path . DIRECTORY_SEPARATOR . $file->getBasename()));
         }
 
         I18nMock::resetPath($path);
diff --git a/tst/JsonApiTest.php b/tst/JsonApiTest.php
index 6de7f7bd..07f33ad2 100644
--- a/tst/JsonApiTest.php
+++ b/tst/JsonApiTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Controller;
diff --git a/tst/MigrateTest.php b/tst/MigrateTest.php
index 81828b62..d587d442 100644
--- a/tst/MigrateTest.php
+++ b/tst/MigrateTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Data\Database;
 use PrivateBin\Data\Filesystem;
diff --git a/tst/ModelTest.php b/tst/ModelTest.php
index 16f3fa6e..04609e22 100644
--- a/tst/ModelTest.php
+++ b/tst/ModelTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use Identicon\Identicon;
 use PHPUnit\Framework\TestCase;
diff --git a/tst/Persistence/PurgeLimiterTest.php b/tst/Persistence/PurgeLimiterTest.php
index 656818ed..d4c8bd5b 100644
--- a/tst/Persistence/PurgeLimiterTest.php
+++ b/tst/Persistence/PurgeLimiterTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Data\Filesystem;
diff --git a/tst/Persistence/ServerSaltTest.php b/tst/Persistence/ServerSaltTest.php
index 99210560..74f535d1 100644
--- a/tst/Persistence/ServerSaltTest.php
+++ b/tst/Persistence/ServerSaltTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Data\Filesystem;
diff --git a/tst/Persistence/TrafficLimiterTest.php b/tst/Persistence/TrafficLimiterTest.php
index 8f7c0e8c..171e56f6 100644
--- a/tst/Persistence/TrafficLimiterTest.php
+++ b/tst/Persistence/TrafficLimiterTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Data\Filesystem;
diff --git a/tst/RequestTest.php b/tst/RequestTest.php
index 6734ba3b..2207fa7e 100644
--- a/tst/RequestTest.php
+++ b/tst/RequestTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Request;
diff --git a/tst/ViewTest.php b/tst/ViewTest.php
index 8043534a..2a1c7e47 100644
--- a/tst/ViewTest.php
+++ b/tst/ViewTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\I18n;
diff --git a/tst/Vizhash16x16Test.php b/tst/Vizhash16x16Test.php
index 63782e51..4ead75fd 100644
--- a/tst/Vizhash16x16Test.php
+++ b/tst/Vizhash16x16Test.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Data\Filesystem;
diff --git a/tst/YourlsProxyTest.php b/tst/YourlsProxyTest.php
index cb34885e..d6e9cb76 100644
--- a/tst/YourlsProxyTest.php
+++ b/tst/YourlsProxyTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 use PHPUnit\Framework\TestCase;
 use PrivateBin\Configuration;