PHP Session保存到数据库

<?php 
class SessionToDB 
{ 
    private $_path         = null; 
    private $_name         = null; 
    private $_pdo          = null; 
    private $_ip           = null; 
    private $_maxLifeTime  = 0; 
     
    public function __construct(PDO $pdo) 
    { 
        session_set_save_handler( 
            array(&$this, 'open'), 
            array(&$this, 'close'), 
            array(&$this, 'read'), 
            array(&$this, 'write'), 
            array(&$this, 'destroy'), 
            array(&$this, 'gc')                                                             
        ); 
         
        $this->_pdo   = $pdo; 
        $this->_ip    = !emptyempty($_SERVER['REMOTE_ADDR']) ? $_SERVER['REMOTE_ADDR'] : null; 
        $this->_maxLifeTime = ini_get('session.gc_maxlifetime'); 
    } 
     
    public function open($path,$name) 
    { 
        return true;     
    } 
     
    public function close() 
    { 
        return true; 
    } 
     
    public function read($id) 
    { 
        $sql = 'SELECT * FROM session where PHPSESSID = ?'; 
        $stmt = $this->_pdo->prepare($sql); 
        $stmt->execute(array($id)); 
         
        if (!$result = $stmt->fetch(PDO::FETCH_ASSOC)) { 
            return null; 
        } elseif ($this->_ip != $result['client_ip']) { 
            return null; 
        } elseif ($result['update_time']+$this->_maxLifeTime < time()){ 
            $this->destroy($id); 
            return null; 
        } else { 
            return $result['data'];     
        } 
    } 
     
    public function write($id,$data) 
    { 
        $sql = 'SELECT * FROM session where PHPSESSID = ?'; 
        $stmt = $this->_pdo->prepare($sql); 
        $stmt->execute(array($id)); 
         
        if ($result = $stmt->fetch(PDO::FETCH_ASSOC)) { 
            if ($result['data'] != $data) { 
                $sql = 'UPDATE session SET update_time =? , date = ? WHERE PHPSESSID = ?'; 
                 
                $stmt = $this->_pdo->prepare($sql); 
                $stmt->execute(array(time(), $data, $id)); 
            } 
        } else { 
            if (!emptyempty($data)) { 
                $sql = 'INSERT INTO session (PHPSESSID, update_time, client_ip, data) VALUES (?,?,?,?)'; 
                $stmt = $this->_pdo->prepare($sql); 
                $stmt->execute(array($id, time(), $this->_ip, $data)); 
            } 
        } 
         
        return true; 
    } 
     
    public function destroy($id) 
    { 
        $sql = 'DELETE FROM session WHERE PHPSESSID = ?'; 
        $stmt = $this->_pdo->prepare($sql); 
        $stmt->execute(array($id)); 
         
        return true;         
    } 
     
    public function gc($maxLifeTime) 
    { 
        $sql = 'DELETE FROM session WHERE update_time < ?'; 
        $stmt = $this->_pdo->prepare($sql); 
        $stmt->execute(array(time() - $maxLifeTime)); 
         
        return true; 
    } 
} 
 
try{ 
    $pdo = new PDO('mysql:host=localhost;dbname=rphp4zf', 'root','rickyfeng'); 
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 
     
    new SessionToDB($pdo);     
} catch(PDOException $e) { 
    echo 'Error: '.$e->getMessage(); 
}