Paypal... they send variables back through POST that lets you know they paid. It's easy enough to add Paypal integration.
Plus, last time I checked, PHP can't process credit cards (god if it could...) so you have to use a 3rd party. Paypal is probaly the most widely used and secure, there's also CCBill and others. If you google for Credit Card Processors, there should be a slew of them. Most will have the option to send a POST variable back to a php page for you to see whether the payment was successful, or if it was denied.
~Brett