Question

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?

Right now I need something that will work with American Express, Discover, MasterCard, and Visa, but it might be helpful if it will also work with other types.

Answer 1

There are three parts to the validation of the card number:

1. PATTERN – does it match an issuers pattern (e.g. VISA/Mastercard/etc.)
2. CHECKSUM – does it actually check-sum (e.g. not just 13 random numbers after ’34’ to make it an AMEX card number)
3. REALLY EXISTS – does it actually have an associated account (you are unlikely to get this without a merchant account)

Pattern

• MASTERCARD Prefix=51-55, Length=16 (Mod10 checksummed)
• VISA Prefix=4, Length=13 or 16 (Mod10)
• AMEX Prefix=34 or 37, Length=15 (Mod10)
• Diners Club/Carte Prefix=300-305, 36 or 38, Length=14 (Mod10)
• Discover Prefix=6011,622126-622925,644-649,65, Length=16, (Mod10)
• etc. (detailed list of prefixes)

Checksum

Most cards use the Luhn algorithm for checksums:

Luhn Algorithm described on Wikipedia

There are links to many implementations on the Wikipedia link, including PHP:

<? /* Luhn algorithm number checker - (c) 2005-2008 shaman - www.planzero.org *  * This code has been released into the public domain, however please      *  * give credit to the original author where possible.                      */  function luhn_check($number) {    // Strip any non-digits (useful for credit card numbers with spaces and hyphens)   $number=preg_replace('/\D/', '', $number);    // Set the string length and parity   $number_length=strlen($number);   $parity=$number_length % 2;    // Loop through each digit and do the maths   $total=0;   for ($i=0; $i<$number_length; $i++) {     $digit=$number[$i];     // Multiply alternate digits by two     if ($i % 2 == $parity) {       $digit*=2;       // If the sum is two digits, add them together (in effect)       if ($digit > 9) {         $digit-=9;       }     }     // Total up the digits     $total+=$digit;   }    // If the total mod 10 equals 0, the number is valid   return ($total % 10 == 0) ? TRUE : FALSE;  } ?>