run processes from php as authenticated user

I'd like to write some web based utilities for users to change things like mail forwarding and vacation autoreplies, as well as password changing. In order to do these things, I need to have a way for the PHP script to assume the privileges of the authenticated user. (If they don't supply the correct username/password, they won't be allowed to use the tools.) Is there any way PHP can do this? I can't just setuid to a static user, since there are many users who would be using this tool. I'm sure there's got to be a way, even if it involves other packages as well. Any ideas? Thanks, -Bill