private static function checksum($number)
{
$number = (string) $number;
$length = strlen($number);
$sum = 0;
for ($i = $length - 1; $i >= 0; $i -= 2) {
$sum += $number[$i];
}
for ($i = $length - 2; $i >= 0; $i -= 2) {
$sum += array_sum(str_split($number[$i] * 2));
}
return $sum % 10;
}