When you are working with an online store, sometimes you need to enter a credit card number that will pass the "logic" of how the card number should be structured so that you can test the flow of your site. These card numbers are not valid for purchase, and if validated against a gateway – it will return as being invalid – however, that's fine if all you're testing is the front end process of the store (before the gateway). In cleaning out my office today, I found a few numbers I used to use for this purpose. I can't remember if all of them worked (and the next time I have to run a test, I'll remove any I use that seem not to) – but wanted to keep these safer than in the dark corners of my office closet…
Visa:
4444 3333 2222 1111
MasterCard:
5424 0000000000 15
(there's 10 zero's in there)
Discover:
6011 0000000000 12
(Again, 10 zero's)
Amex:
37 000000000000 2
(12 zero's in there)
There's also a bunch of other test credit card numbers here in case you need them.