How can PHP be used to include files from a parent directory when accessing a subdomain?

When accessing a subdomain in PHP, you may encounter issues with including files from a parent directory due to the file path resolution. To solve this, you can use the `$_SERVER['DOCUMENT_ROOT']` variable to get the root directory of the server and then navigate to the parent directory using `../` in the file path.

<?php
include $_SERVER['DOCUMENT_ROOT'] . '/../filename.php';
?>