PHP随機獲得一個字元串

/**
     * 擷取随機字元串
     * @param int $len 字元串長度
     * @param string $chars 字元源
     * @return string
     */
    function getRandomString($len, $chars = null)
    {
        if (is_null($chars)) 
            $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        
        for ($i = 0, $str = '', $lc = strlen($chars) - 1; $i < $len; $i++) {
            $str .= $chars[mt_rand(0, $lc)];
        }
        return $str;
    }