performing tasks with a user's rights

Is there any way for a php script to perform some task as a particular user? Here's what I'm trying to do in particular: I have a Linux box with a handful of users. These users do not have shell access, but they do have ftp (for file uploads to their web sites), pop, imap, etc. I'd like to provide an easy way for them to change their passwords (at least, and possibly also edit some files like their aliases file (virtual mail hosting) and so on). To do this through a web interface, I want first of all to authenticate them, then to get their new password, say, from a form submittal, and run passwd. Any suggestions? Do I have to run apache as root? (Shudder!) Are there packages out there that do this already, or that I could use as an example? TIA. Nate