Apache config / redirect / forward / proxy solution

There are 2 existing websites - website1 and website2 hosted on same host. Website1 is application website for user register, website2 is for user payments. Payment systems are ready and installed on website1. I am looking for this solution: 1. User checkout at website1 2. Payment get proxied/redirected/forwarded internally to website2 (without user interaction) 3. from website2 it goes to payment provider i want the visibility of webite2 to be seen on http_ref and source_url to the payment gateway. All must be done on the server side, not open new window for user. looking for easy and lightweight solution. Please only experienced users who know what they doing. thank you