I don't like to give you wrong information and I don't have time to make test now... but I think that it is about URL rewriting...
I think this fonction replace URL like http://yourdomain.com/profile.php?id=1234 with http://yourdomain.com/UserName
The URL rewriting rules could be different for your hosting. In this case the .htaccess file needs to be edited. Often the URL rewriting rules can be found in the FAQ of your hosting provider...