public function randomStr( $length = 16 )
    {
        $arr = array_merge(range(0, 9), range('a', 'z'), range('A', 'Z'));

        $str = '';
        $arr_len = count($arr);
        for ($i = 0; $i < $length; $i++)
        {
            $rand = mt_rand(0, $arr_len-1);
            $str.=$arr[$rand];
        }

        return $str;
    }