running a php script as a user

Okay, I'll try to make this short. I work for a hosting provider, and thustly we have a few hundred clients. We have been working to provide a web-based file management system for the users, but have come across a stumbling block. The problem is allowing the users read/write/delete/move/create access to their folders on the server. We want to be able to run a central php script, but be able to dynamically tell that script what user to run as, based on HTTP authentication (username & password provided). I've seen plenty of documentation about running scripts that use the suid bit, but those scripts are required to be owned by the person who the script is to be run as. Any help is greatly appreciated, Mike