session写入数据库

 <?php
     class session
     {
         private static $handle = null;
         private static $ip = null;
         private static $lifetime = null;
         private static $time = null;

         static function init($pdo)
         {
             self::$handle = $pdo;
             self::$ip = !empty($_SERVER["REMOTE_ADDR"]) ? $_SERVER["REMOTE_ADDR"] : "unknow";
             self::$lifetime = ini_get('session.gc_maxlifetime');
             self::$time = time();
         }

         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;
         }

         public static function read($PHPSESSID)
         {
             $sql = "select * from session where PHPSESSID = ?";
             $smit = self::$handle->prepare($sql);
             $smit->execute(array($PHPSESSID));

             if(!$result = $smit->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'];
         }

         public static function write($PHPSESSID,$data)
         {
             $sql = "select * from session where PHPSESSID = ?";
             $stmt = self::$handle->prepare($sql);
             $stmt->execute(array($PHPSESSID));

             if($result = $stmt->fetch(PDO::FETCH_ASSOC))
             {
                 if($result['data'] != $data || self::$time-30 > $result['update_time'])
                 {
                     $sql="update session set update_time = ?, data =? where PHPSESSID = ?";
                     $stmt = self::$handle->prepare($sql);
                     $stmt->execute(array(self::$time,$data,$PHPSESSID));
                 }

             }else
             {
                 if(!empty($data))
                 {
                     $sql="insert into session(PHPSESSID, update_time, client_ip, data) values(?,?,?,?)";
                     $sth = self::$handle->prepare($sql);
                     $sth->execute(array($PHPSESSID,self::$time,self::$ip,$data));
                 }
             }
             return true;
         }

         public static function destroy($PHPSESSID)
         {
             $sql = "delete from session where PHPSESSID = ?";
             $sth = self::$handle->prepare($sql);
             $sth->execute(array($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-self::$lifetime));
             return true;
         }
     }

     try{
         $pdo = new PDO("mysql:host=localhost;dbname=test", "root" ,"");
     }catch(PDOException $e)
     {
         echo $e->getMessage();
     }

     session::start($pdo);