PHP

luhn algorithm

<?php
function is_valid_luhn($number) {
  settype($number, 'string');
  $sumTable = array(
    array(0,1,2,3,4,5,6,7,8,9),
    array(0,2,4,6,8,1,3,5,7,9));
  $sum = 0;
  $flip = 0;
  for ($i = strlen($number) - 1; $i >= 0; $i--) {
    $sum += $sumTable[$flip++ & 0x1][$number[$i]];
  }
  return $sum % 10 === 0;
}

luhn algorithm

function validateLuhn(string $number): bool
{
    $sum = 0;
    $flag = 0;

    for ($i = strlen($number) - 1; $i >= 0; $i--) {
        $add = $flag++ & 1 ? $number[$i] * 2 : $number[$i];
        $sum += $add > 9 ? $add - 9 : $add;
    }

    return $sum % 10 === 0;
}