setting user id?

can i set the userID of my script so that it can read files that arent chmod public? here's the code i'm working with.... Header ( "Content-Type: application/octet-stream"); Header ( "Content-Length: ".filesize($url)); Header( "Content-Disposition: attachment; filename=$song_name"); readfile($url); if I chmod the file ($url) so it can be publically read it works fine, if its not public it downloads a file with the line (permission denied) -lelon