$numbers = "49927398716 49927398717 1234567812345678 1234567812345670";
foreach (split(' ', $numbers) as $n)
  echo "$n is ", luhnTest($n) ? 'valid' : 'not valid', '</br>';
 
function luhnTest($num) {
  $len = strlen($num);
  for ($i = $len-1; $i >= 0; $i--) {
    $ord = ord($num[$i]);
    if (($len - 1) & $i) {
      $sum += $ord;
    } else {
      $sum += $ord / 5 + (2 * $ord) % 10;
    }
  }   
  return $sum % 10 == 0;
}

49927398716 is valid
49927398717 is not valid
1234567812345678 is not valid
1234567812345670 is valid

49927398716 is valid
49927398717 is not valid
1234567812345678 is not valid
1234567812345670 is valid