Note: Don't go pasting a customer's credit card willy nilly all over the internet. While the links on this page may offer example pages, DO NOT TRUST THEM, as a general rule. Copy them to your own computer or server and test them there, after making sure they're not doing anything nefarious.

If a customer complains that their card isn't being accepted, you may want to run it through some basic validation tests. The most simple is the check digit, which is the last digit of the card. The math isn't too bad, but the basic idea is that the last digit is generated from the preceding digits. If it's off, the card is invalid.

• Check Digit test, and example