php将文件类型session写入memcache

<?php    
    class MemSession{
        private static $handler = null;
        private static $lifetime = null;
        private static $time = null;
        const MS = 'session';
        
        private static function init($handler){
            self::$handler = $handler;
            self::$lifetime = ini_get('session.gc_maxlifetime');
            self::$time = time();
        }
        
        public static function start($memcache){
            self::init($memcache);
            //调用类中的方法要用数组,__CLASS__代表本类
            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){
            
        }
        public static function close(){
            
        }
        
        public static function read($PHPSESSID){
            $val = self::$handler->get(self::session_key($PHPSESSID));
            
            if($val===false || $val==null){
                return false;
            }
            return $val;
        }
        public static function write($PHPSESSID,$data){
            $method = $data? 'set':'replace';
            return self::$handler->$method(self::session_key($PHPSESSID),$data,MEMCACHE_COMPRESSED,self::$lifetime);
        }
        
        public static function destroy($PHPSESSID){
            return self::$handle->delete(self::session_key($PHPSESSID));
        }
        //memcache本身就有限定时间，数据自动销毁，所以可不使用gc方法
        public static function gc($lifetime){
            return true;
        }
        
        //给sessionID加前缀，避免key重复
        private static function session_key($PHPSESSID){
            $session_key = self::MS.$PHPSESSID;
            return $session_key;
        }        
    }
    $mem = new Memcache;
    $mem->connect("localhost",11211) or die("could not connect");
    MemSession::start($mem);