protected function randomMixed()
    {
        $characters = implode('', $this->letters) . $this->chineseCharacters;

        if (function_exists('mb_substr'))
        {
            $len = mb_strlen($characters, 'utf-8');

            for ($i = 0; $i < $this->length; $i++)
            {
                $this->code .= mb_substr($characters, mt_rand(0, $len - 1), 1, 'utf-8');
            }
        }
    }