Payment processing is a major thing that has huge legal and security concerns. You can't run a PayPal-like system with a pre-made script. You can code a shopping cart, but the actual payment processing requires a merchant account that connects to the major credit cards.
Plus, a Poker program that's multiplayer would probably need some kind of client-side technology, such as Flash or Ajax to be up to date. PHP by itself would be too slow and require constant reloading.
(Sorry this post isn't really... encouraging.)