Personally, I would just use a wallet type site that accepts cards or something as it would be more secure. Yeah writing your own is fun/cool and all, but it can be a bit insecure if you are unsure how to ensure its security.

I've never approached using a card on a website, so I cannot provide advice for you there. But what I can help with is what I do when I need to accept cards. I usually just use PayPal since they can use any type of card (even without an account).

Nonetheless, I've seen tutorials online about integrating cards being used with PHP. A simple [ Register or Signin to view external links. ] search should return a plethora of results.