What is the best way to validate a credit card in PHP?

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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;

}
?>

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