Cannot access outside of domian folder using __DIR__

while I was using __DIR__ . '/../folder/outside_of_domain_folder/file.inc.php to access a file outside of my domain folder, an error was returned (no such file or directory) but the file does exist. But when I used it access a include file within my domain folder, __DIR__ works again. Any reasons why this happened? Or did I made some mistake in my code?