Credit card number checker?

I know that credit cards have standard formats: There's a standard number of digits, and whether the card is Visa, MC, Amex, etc is encoded in the number, and there is some kind of checksum, and I think the expiration is also encoded in the number. All of this is obvious stuff that anyone designing such a system would do. I'm wondering if anyone can refer me to a site that describes what this format is, so I can write some PHP code that will check to see if a credit card number format is correct. I don't even want to try to run the card through my merchant account if the format is obviously wrong. I assume that banks check the rejection rate on their merchant accounts, and too many bogus cards would not look good. I did a quick search on the web, and there are a vast number of "hacker" credit card number generators, but that isn't exactly what I'm looking for. Thanks