Accessing and sending commands to a pop account was one of the most difficult things I have done in PHP. There is an object out there that I found that did most of the hard work for me, but even using that object got hairy at times.

Try these resources to see where they lead you
http://www.php.net/imap
http://www.phpbuilder.com/manual/ref.imap.php

Good luck, when I tried this a few years back I got a migraine after a few days of bad programming and grabbed an out of the box solution.