Credit Card Verification VIA LUHN

cardverification.php

/* Credit Card Verification VIA LUHN by serpico
function card_verification($ccn){
        $ccn = str_replace (' ', '', str_replace ('-', '', trim($ccn)));
        if(strlen($ccn) != 16 && !is_numeric($ccn)){
            return false;
        }
        $checksum = 0;                                  // running checksum total
        $j = 1;
        for ($i = strlen($ccn) - 1; $i >= 0; $i--) {
          // Extract the next digit and multiply by 1 or 2 on alternative digits.
          $calc = $ccn{$i} * $j;
          // If the result is in two digits add 1 to the checksum total
          if ($calc > 9) {
            $checksum = $checksum + 1;
            $calc = $calc - 10;
          }

          // Add the units element to the checksum total
          $checksum = $checksum + $calc;

          // Switch the value of j
          if ($j ==1) {$j = 2;} else {$j = 1;};
        }
        if ($checksum % 10 != 0) {
         return false;
        }

        
        return $ccn;
    }