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The integrity of the data can be checked at any later time by recomputing the checksum and comparing it with the stored one.
If the checksums match, the data was almost certainly not altered.
Given a credit card number and no additional information, what is the best way in PHP to determine whether or not it is a valid number?
Step 3: The total obtained in Step 2 must be a number ending in zero (30, 40, 50, etc.) for the account number to be validated.
The old version of i Tunes might be creating trouble.
Send the card info right over to your payment gateway and then deal with their response.
It helps them detect fraud if you don't do anything like Luhn checking first -- let them see the failed attempts.