diff --git a/system/CodeIgniter.php b/system/CodeIgniter.php
index 359e3ff7d08d..6d5c55be32eb 100644
--- a/system/CodeIgniter.php
+++ b/system/CodeIgniter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This file is part of CodeIgniter 4 framework.
  *
diff --git a/system/Config/BaseConfig.php b/system/Config/BaseConfig.php
index 8a82cbf7ce64..cc128b131873 100644
--- a/system/Config/BaseConfig.php
+++ b/system/Config/BaseConfig.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This file is part of CodeIgniter 4 framework.
  *
diff --git a/system/Test/ControllerTestTrait.php b/system/Test/ControllerTestTrait.php
index 5a5bc708692f..7c94aa6e128f 100644
--- a/system/Test/ControllerTestTrait.php
+++ b/system/Test/ControllerTestTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * This file is part of CodeIgniter 4 framework.
  *