NEUBER-WEB-SOLUTIONS :: We support your ECommerce | What Credit Card numbers can/should tell us

How can one make sure that a provided credit card number is valid?
There might be the situation that a customer wants to provide you the number by telephone.
Although checking CC numbers is a science on it’s own, there are some basic rules good to keep in mind:

Starting digit:
Every credit card type has it’s fixed starting digit – good to know:

Amex/Diners Club: 3
VisaCard: 4
Mastercard: 5
Discover: 6

Length of number:
Also the length of the number is fixed according to card type:

DinersClub: 13
Amex: 15
All others: 16

So, the customer wants to pay with his VISA Card, but he provided only 15 digits?
And even worse, the first digit was a 7?
No, we don’t think so…

Luhn or Mod10 algorithm – the CC checksum has to be right
This checksum algorithm was invented by Hans Luhn. If the generated checksum can be divided by 10 without rest – then the number is valid – so the basic idea.
Here a practical PHP function to validate a credit card numer with it:

function VerifyLuhn($number) {
   $split = array_reverse(str_split($number));
    for ($i=1;$i<=count($split);$i+=2) {
        if (isset($split[$i])) $split[$i] = array_sum(str_split($split[$i]*2));
    }
   return (array_sum($split) % 10) ? FALSE : TRUE;
}

Warning: Using “Online Credit card number verifiers” is potentially dangerous! Somebody might “harvest” valid credit numbers and misuse them.

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Comments, improvments are welcome!