public static function randomString(int $length = 10, string $letters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ') : string
    {
        return substr(str_shuffle(str_repeat($x = $letters, ceil($length / strlen($x)))), 1, $length);
    }