validate_creditcard.php

<?php

/*
 * Takes a credit card number and ensures
 * it contains the proper number of digits
 * for the given card type.
 */
function isValidCreditCardNumber($cardNumber)
{
  // First check that the card number 
  // is a valid length.
  $firstNumber = substr($cardNumber, 0, 1);
  switch ($firstNumber)
  {
    case 3:
      // American Express
      if (!preg_match('/^3\d{3}[ \-]?\d{6}[ \-]?\d{5}$/', $cardNumber))
      {
        return false;
      }
      break;

    case 4:
      // Visa
      if (!preg_match('/^4\d{3}[ \-]?\d{4}[ \-]?\d{4}[ \-]?\d{4}$/', $cardNumber))
      {
        return false;
      }
      break;

    case 5:
      // MasterCard
      if (!preg_match('/^5\d{3}[ \-]?\d{4}[ \-]?\d{4}[ \-]?\d{4}$/', $cardNumber))
      {
        return false;
      }
      break;

    case 6:
      // Discover
      if (!preg_match('/^6011[ \-]?\d{4}[ \-]?\d{4}[ \-]?\d{4}$/', $cardNumber))
      {
        return false;
      }
      break;

    default:
      return false;
  }

  // All credit cards issued today are based on 
  // a modulus 10 algorithm, so we can use the 
  // Luhn Algorithm. 
  $cardNumber = str_replace('-', '', $cardNumber);

  $map = array(0, 1, 2, 3, 4, 5, 6, 7, 8, 9,
              0, 2, 4, 6, 8, 1, 3, 5, 7, 9);
  $sum = 0;
  $last = strlen($cardNumber) - 1;
  for ($i = 0; $i <= $last; $i++)
  {
    $sum += $map[$cardNumber[$last - $i] + ($i & 1) * 10];
  }

  if ($sum % 10 != 0)
  {
    // Invalid card number
    return false;
  }

  return true;
}

isValidCreditCardNumber('4111-1111-1111-1111'); // True
isValidCreditCardNumber('5558-545f-1234'); // False
isValidCreditCardNumber('9876-5432-1012-3456'); // False

?>