diff --git a/system/Common.php b/system/Common.php
index 0483fd5934..9256ff1cf6 100644
--- a/system/Common.php
+++ b/system/Common.php
@@ -725,8 +725,18 @@ if (! function_exists('is_windows')) {
     /**
      * Detect if platform is running in Windows.
      */
-    function is_windows(): bool
+    function is_windows(?bool $mock = null): bool
     {
+        static $mocked;
+
+        if (func_num_args() === 1) {
+            $mocked = $mock;
+        }
+
+        if (isset($mocked)) {
+            return $mocked;
+        }
+
         return DIRECTORY_SEPARATOR === '\\';
     }
 }
diff --git a/tests/system/CommonFunctionsTest.php b/tests/system/CommonFunctionsTest.php
index 0049973f36..2e8c3be0d6 100644
--- a/tests/system/CommonFunctionsTest.php
+++ b/tests/system/CommonFunctionsTest.php
@@ -706,4 +706,19 @@ final class CommonFunctionsTest extends CIUnitTestCase
         $this->assertSame(strpos(php_uname(), 'Windows') !== false, is_windows());
         $this->assertSame(defined('PHP_WINDOWS_VERSION_MAJOR'), is_windows());
     }
+
+    public function testIsWindowsUsingMock()
+    {
+        is_windows(true);
+        $this->assertTrue(is_windows());
+        $this->assertNotFalse(is_windows());
+
+        is_windows(false);
+        $this->assertFalse(is_windows());
+        $this->assertNotTrue(is_windows());
+
+        is_windows(null);
+        $this->assertSame(strpos(php_uname(), 'Windows') !== false, is_windows());
+        $this->assertSame(defined('PHP_WINDOWS_VERSION_MAJOR'), is_windows());
+    }
 }
diff --git a/user_guide_src/source/general/common_functions.rst b/user_guide_src/source/general/common_functions.rst
index 1a3a4f2990..b9ed2317c7 100755
--- a/user_guide_src/source/general/common_functions.rst
+++ b/user_guide_src/source/general/common_functions.rst
@@ -281,12 +281,19 @@ Miscellaneous Functions
     :returns: true if you can write to the file, false otherwise.
     :rtype: bool
 
-.. php:function:: is_windows()
+.. php:function:: is_windows([$mock = null])
 
+    :param bool|null $mock: If given and is a boolean then it will be used as the return value.
     :rtype: bool
 
     Detect if platform is running in Windows.
 
+    .. note:: The boolean value provided to $mock will persist in subsequent calls. To reset this
+        mock value, the user must pass an explicit ``null`` to the function call. This will
+        refresh the function to use auto-detection.
+
+    .. literalinclude:: common_functions/012.php
+
 .. php:function:: log_message($level, $message [, $context])
 
     :param   string   $level: The level of severity
diff --git a/user_guide_src/source/general/common_functions/012.php b/user_guide_src/source/general/common_functions/012.php
new file mode 100644
index 0000000000..7c716dc0b3
--- /dev/null
+++ b/user_guide_src/source/general/common_functions/012.php
@@ -0,0 +1,11 @@
+<?php
+
+is_windows(true);
+
+// some code ...
+
+if (is_windows()) {
+    // do something ..
+}
+
+is_windows(null); // reset