authenticating access from intranet to internet

Is it possible with PHP under Linux and if so, what would it take, to create a web proxy server with authentication so that all attempts at unauthenticated access to the internet are blocked and the user is shown a web page with a message when attempting to access directly without authentication? (I'm using IPCHAINS and IPMASKADM already, but no proxy.) The idea is to provide a web page which allows folks to register (with a credit card transaction) to use our gateway service to access the internet from this country's intranet. Other companies are doing similar service, but they are mostly using out of the box software on Wintel boxes. Ken