I would like to be able to run PHP commands, in system-function exec or whatever, but with a different user then www-data. 
OS: Ubuntu Server 10.10
Web Server: Apache2

I've searched for answers a couple of hours now, but it's a dead end. Most people says it's impossible and others don't know. I would like to run commands with a different user in PHP. This is kind of what I want:
User presses a button that starts a screen running CS Source server. The screen is run from myuser.

Hope you'll understand what I want here. Also I don't know what function to use (system, exec or other) so some tips here as well could be usefull. 

Thanks in advance!