diff --git a/system/CodeIgniter.php b/system/CodeIgniter.php
index 368b6130cf..b7a6f4ae5d 100644
--- a/system/CodeIgniter.php
+++ b/system/CodeIgniter.php
@@ -1,7 +1,5 @@
 <?php
 
-declare(strict_types=1);
-
 /**
  * This file is part of CodeIgniter 4 framework.
  *
@@ -921,6 +919,8 @@ class CodeIgniter
         // This is a Web request or PHP CLI request
         $params = $this->router->params();
 
+        // The controller method param types may not be string.
+        // So cannot set `declare(strict_types=1)` in this file.
         $output = method_exists($class, '_remap')
             ? $class->_remap($this->method, ...$params)
             : $class->{$this->method}(...$params);
diff --git a/system/Config/BaseConfig.php b/system/Config/BaseConfig.php
index ed6fa3e2bc..6789c0e9c7 100644
--- a/system/Config/BaseConfig.php
+++ b/system/Config/BaseConfig.php
@@ -1,7 +1,5 @@
 <?php
 
-declare(strict_types=1);
-
 /**
  * This file is part of CodeIgniter 4 framework.
  *
@@ -166,6 +164,9 @@ class BaseConfig
                 $value = (float) $value;
             }
 
+            // If the default value of the property is `null` and the type is not
+            // `string`, TypeError will happen.
+            // So cannot set `declare(strict_types=1)` in this file.
             $property = $value;
         }
     }
diff --git a/system/Test/ControllerTestTrait.php b/system/Test/ControllerTestTrait.php
index 18a645d685..af23d73f2b 100644
--- a/system/Test/ControllerTestTrait.php
+++ b/system/Test/ControllerTestTrait.php
@@ -1,7 +1,5 @@
 <?php
 
-declare(strict_types=1);
-
 /**
  * This file is part of CodeIgniter 4 framework.
  *
@@ -163,6 +161,8 @@ trait ControllerTestTrait
 
         try {
             ob_start();
+            // The controller method param types may not be string.
+            // So cannot set `declare(strict_types=1)` in this file.
             $response = $this->controller->{$method}(...$params);
         } catch (Throwable $e) {
             $code = $e->getCode();
diff --git a/system/View/Parser.php b/system/View/Parser.php
index 1a684a1d1e..dc34a74504 100644
--- a/system/View/Parser.php
+++ b/system/View/Parser.php
@@ -1,7 +1,5 @@
 <?php
 
-declare(strict_types=1);
-
 /**
  * This file is part of CodeIgniter 4 framework.
  *
@@ -618,6 +616,7 @@ class Parser extends View
             }
 
             // Filter it....
+            // We can't know correct param types, so can't set `declare(strict_types=1)`.
             $replace = $this->config->filters[$filter]($replace, ...$param);
         }
 
diff --git a/tests/system/Debug/ExceptionsTest.php b/tests/system/Debug/ExceptionsTest.php
index 7285d3e435..3f45399c3c 100644
--- a/tests/system/Debug/ExceptionsTest.php
+++ b/tests/system/Debug/ExceptionsTest.php
@@ -1,7 +1,5 @@
 <?php
 
-declare(strict_types=1);
-
 /**
  * This file is part of CodeIgniter 4 framework.
  *
@@ -73,6 +71,7 @@ final class ExceptionsTest extends CIUnitTestCase
         $maybeNull = PHP_VERSION_ID >= 80100 ? null : 'random string';
 
         try {
+            // We test DEPRECATED error, so cannot set `declare(strict_types=1)` in this file.
             strlen($maybeNull);
             $this->assertLogContains('error', '[DEPRECATED] strlen(): ');
         } catch (ErrorException $e) {