PHP saying folder does not exist

This PHP code is supposed to check if a folder exists based on the value of the URL perimeter ?username=

 <?php
    $username = $_GET["username"];
    $filename = "http://officedocs.freeserver.me/userdata/" . $username . "/docs.xml";

    if (file_exists($filename)) {
    header("Location: http://nsc-component.webs.com/Office/auth.html?username=".$username);
    } else {
    header("Location: http://nsc-component.webs.com/Office/user/login.html?error=2");
    }
    ?>

But it's saying the folder doesn't exist even though it does.

I'm not a PHP wiz I found this code and modded it. Is there anything wrong with the PHP?