public static function isCreditcard($value)
{
$length = strlen($value);
if ($length < 13 || $length > 19) {
return false;
}
$sum = 0;
$weight = 2;
for ($i = $length - 2; $i >= 0; $i--) {
$digit = $weight * $value[$i];
$sum += floor($digit / 10) + $digit % 10;
$weight = $weight % 2 + 1;
}
$mod = (10 - $sum % 10) % 10;
return $mod == $value[$length - 1];
}