public function string($length)
    {
        $chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
        $str = '';
        for ($i = 0; $i < $length; ++$i) {
            $str .= substr($chars, mt_rand(0, strlen($chars) - 1), 1);
        }

        return $str;
    }