squid proxy authentication

Hi, I am looking to set up a way to authenticate users behind a NATting router via squid (or any other proxy or solution) using a PHP splash page that asks for login and password and checks them via a DB. I saw squid has some kind of session based authentication which i haven't managed to get to work, but I'm assuming it can be made to. However my problem is the NAT part. Need to somehow maintain the squid session ID on a per-user basis and not per-ip (that will just allow everyone through once one person enters their login). Please provide a solution or proof of concept setup that works. Once you can get this authentication problem sorted out, we'll discuss the rest of the project which is quite large. NO SPAM MESSAGES. I DON'T CARE IF YOU MADE 1000 JOOMLA SITES OR WORDPRESS BLOGS.