<"REMOTE_ADDR"])"REMOTE_ADDR"]:'unkonw';    //获取客户端ip
      self::$lifetime = ini_get('session.gc_maxlifetime');    //获取session生命周期
      self::$time = time();    //获取当前时间
    }
    //调用session_set_save_handler()函数并开启session
    static function start($pdo){
      self::init($pdo);
      session_set_save_handler(
        array(__CLASS__,'open'),
        array(__CLASS__,'close'),
        array(__CLASS__,'read'),
        array(__CLASS__,'write'),
        array(__CLASS__,'destroy'),
        array(__CLASS__,'gc')
      );
      session_start();
    }
    
    public static function open($path,$name){
      return true;
    }
    public static function close(){
      return true;
    }
    
    //查询数据库中的数据
    public static function read($PHPSESSID){
      $sql = "select PHPSESSID,update_time,client_ip,data from session where PHPSESSID=";
      $stmt = self::$handler->prepare($sql);
      $stmt->execute(array($PHPSESSID));
      if(!$result = $stmt->fetch(PDO::FETCH_ASSOC)){
        return '';
      }
      if(self::$ip == $result['client_ip']){
        self::destroy($PHPSESSID);
        return '';
      }
      if(($result['update_time']+self::$lifetime)<self::$time){
        self::destroy($PHPSESSID);
        return '';
      }
      return $result['data'];
    }
    /*
    *首先查询该session是否存在数据，如果存在，则更新数据，如果不存在，则插入数据
    */
    //将session写入数据库中，$data传入session中的keys和values数组
    public static function write($PHPSESSID,$data){
      $sql = "select PHPSESSID,update_time,client_ip,data from session where PHPSESSID=";
      $stmt = self::$handler->prepare($sql);
      $stmt->execute(array($PHPSESSID));
      
      if($result=$stmt->fetch(PDO::FETCH_ASSOC)){        
        if($result['data'] != $data || self::$time > ($result['update_time']+30)){
          $sql = "update session set update_time=";
          $stmt = self::$handler->prepare($sql);
          $stmt->execute(array($self::$time,$data,$PHPSESSID));
        }
      }else{
        if(!empty($data)){
          try{
            $sql = "insert into session(PHPSESSID,update_time,client_ip,data) values(";
          }catch(PDOException $e){
            echo $e->getMessage();
          }
          $sth = self::$handler->prepare($sql);
          $sth->execute(array($PHPSESSID,self::$time,self::$ip,$data));
        }
      }
      return true;
    }
    
    public static function destroy($PHPSESSID){
      $sql = "delete from session where PHPSESSID = ";
      $stmt = self::$handler->prepare($sql);
      $stmt->execute(array($PHPSESSID));
      return true;
    }
    public static function gc($lifetime){
      $sql = "delete from session where update_time<";
      $stmt = self::$handler->prepare($sql);
      $stmt->execute(array(self::$time-$lifetime));
      return true;
    }
  }
  //使用PDO连接数据库
  try{
    $pdo = new PDO("mysql:host=localhost;dbname=session","root","hwj193");
  }catch(PDOException $e){
    echo $e->getMessage();
  }
  //传递数据库资源
  Session::start($pdo);