session保存在mysql

<?php
require_once("class_mysql.php");

class session
{
    public $db;
    function session(&$db)
    {
        $this -> db = &$db;
        session_module_name('user');
        session_set_save_handler(
            array(&$this, 'open'),
            array(&$this, 'close'),
            array(&$this, 'read'),
            array(&$this, 'write'),
            array(&$this, 'destroy'),
            array(&$this, 'gc')
            );
        session_start();
    } 
    function unserialize($data_value)
    {
        $vars = preg_split('/([a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*)\|/', $data_value, -1, 
        PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE
            );
        for ($i = 0; isset($vars[$i]); $i++)
        {
            $result[$vars[$i++]] = unserialize($vars[$i]);
        } 
        return $result;
    } 
    function open($path, $name)
    {
        return true;
    } 
    function close()
    {
        return true;
    } 
    function read($session_id)
    {
        $session_id = $this -> db -> escape_string($session_id);
        if ($row = $this -> db -> query("select * from `sessions` where `session_id` = '$session_id' limit 1"))
        {
            return $row['data_value'];
        } 
        else
        {
            $this -> db -> query("insert into `sessions` set `session_id` = '$session_id'");
            return "";
        } 
    } 
    function write($session_id, $data_value)
    {
        $data = $this -> unserialize($data_value);
        $session_id = $this -> db -> escape_string($session_id);
        $data_value = $this -> db -> escape_string($data_value);
        $sql = "update `sessions` set ";
        if (isset($data['user_id']))
        {
            $sql .= "`user_id` = '{$data['user_id']}', ";
        } 
        $sql .= "`data_value` = '$data_value', "
         . "`last_visit` = null "
         . "where `session_id` = '$session_id'";
        $this -> db -> query($sql);
        return true;
    } 
    function destroy($session_id)
    {
        $session_id = $this -> db -> escape_string($session_id);
        $this -> db -> query("delete from `sessions` where `session_id` = '$session_id'");
        return true;
    } 
    function gc($lifetime)
    {
       $this -> db -> query("delete from `sessions` 
       where unix_timestamp(now()) - unix_timestamp(`last_visit`) > $lifetime");
        return true;
    } 
    // get sessions by user_id
    function get($user_id)
    {
        $user_id = $this -> db -> escape_string($user_id);
        return $this -> db -> query("select * from `sessions` where `user_id` = '$user_id'");
    } 
    // get sessions list
    function lists($page, $rows)
    {
        if ($page == 0)
        {
            return $this -> db -> query("select * from `sessions` order by `user_id`");
        } 
        else
        {
            $start = ($page - 1) * $rows;
            return $this -> db -> query("select * from `sessions` order by `user_id` limit $start, $rows");
        } 
    } 
} 

?>