Luhn algorithm

Here's the Luhn algorithm in PHP. The Luhn algorithm is mainly used to check if a credit/debit card number is valid or not. Please tell me if there's a bug somewhere, I've tested the thing heavily and it's working like a charm. /* *@function check_luhn *@desc This function checks if a card number follows the Luhn algorithm *@param string $strCreditCardNumber The card number (maybe AMEX, Diner's Club, Discover, JCB, Maestro, MasterCard, Solo, Switch, Visa, and Visa Electron) *@return boolean TRUE if the card follows the Luhn algorith, false if not *@usage check_lun('4123456789012345') */ function check_luhn($strCardNumber){ //first convert the credit card number to an array of digits $arrCardDigits = array(); for ($i = 0; $i < strlen($strCardNumber); $i++) $arrCardDigits[$i] = (int)$strCardNumber[$i]; //now start the algorithm $sum = 0; $alt = false; for($i = count($arrCardDigits) - 1; $i >= 0; $i--){ if($alt){ $arrCardDigits[$i] *= 2; if($arrCardDigits[$i] > 9) $arrCardDigits[$i] -= 9; } $sum += $arrCardDigits[$i]; $alt = !$alt; } return $sum % 10 == 0; } -- itoctopus - http://www.itoctopus.com