On the PHP::Impact blog Federico has posted a sort of reminder about a method you can use to run PHP scripts as different users on the same system - suPHP.

suPHP is a tool for executing PHP scripts with the permissions of their owners. It consists of an Apache module (mod_suphp) and a setuid root binary (suphp) that is called by the Apache module to change the uid of the process executing the PHP interpreter.

suPHP runs as an Apache module and works with the local server's permission system to restrict script access to whatever the user's allowances are. He links to this tutorial for more information on setting it up.