Sorting Issue With Incremental Content Plugin

I'm using the following code to sort incremental content by number (Week 1, Week 2, Week 3, etc) and it has worked fine until I hit Week 10 then it started ordering it like this: Week 1, Week 10, Week 11, Week 2, Week 3, etc.) I can't for the life of me figure out how to fix this and was hoping someone here could help me out?

PHP:

<?php
session_start();
function cmp($a, $b)
{
if ($a['link_url'] == $b['link_url']) {
return 0;
}
return ($a['link_url'] < $b['link_url']) ? -1 : 1;
}

if ($_SESSION['_amember_links'])
{
usort($_SESSION['_amember_links'],'cmp');
foreach ($_SESSION['_amember_links'] as $link_id => $link) 
    {    
        if($link['link_group_id'] == "0" && preg_match("/\bIntensive\b/i", $link['link_title'])) {
        echo "<a href=\"".$link['link_url']."\"><font face=\"Arial\" size=\"5\">".$link['link_title']."</font></a><br><br>";
        }
    }
}
?>


Rename the content to Week 01, Week 02...

Already tried that it doesn't work either. Still does the same thing...weird?