/**
     * 
     * 产生随机字符串，不长于32位
     * @param int $length
     * @return 产生的随机字符串
     */
    public static function getNonceStr($length = 32) 
    {
        $chars = "abcdefghijklmnopqrstuvwxyz0123456789";  
        $str ="";
        for ( $i = 0; $i < $length; $i++ )  {  
            $str .= substr($chars, mt_rand(0, strlen($chars)-1), 1);  
        } 
        return $str;
    }