session_set_save_handler()中write使用的有关问题

<?php
  require('session.php');

  session_start();
  $_SESSION['test'] = 1;

  echo '<a href="b.php?' . session_name() . '=' . session_id() . '">查看</a>';

<?php
  require('session.php');
  session_start();

<?php
    ini_set('session.save_handler', 'user');
    ini_set('session.use_cookies', 0);
    
    session_set_save_handler('open', 'close', 'read', 'write', 'destroy', 'gc');
    
    $link = null;
    $expiry = 86400;
    
    function open($save_path, $session_name)
    {
        //连接数据库
    }
    
    function close() {
        return true;
    }
    
    function read($sessionId) {
        global $link;
        
        $sessionId = mysql_escape_string($sessionId);
        $time = time();
        $sql = "SELECT value FROM session WHERE session_id='{$sessionId}' AND expiry>{$time}";
        
        $result = mysql_query($sql, $link);
        
        if(@$row = mysql_fetch_row($result, $link)) {
            return $row[0];
        }else {
            return '';
        }
    }
    
    function write($sessionId, $sessionData) {
        global $link, $expiry;
        
        $sessionId = mysql_escape_string($sessionId);
        $sessionData = mysql_escape_string($sessionData);
        $time = time() + $expiry;
        
        $sql = "INSERT INTO session(session_id, value, expiry) VALUES('{$sessionId}', '{$sessionData}', {$time});";
        $result = mysql_query($sql, $link);
        
        if($result) {
            return true;
        }
        
        $sql = "UPDATE session SET value='{$sessionData}', expiry={$time} WHERE session_id='{$sessionId}'";
        $result = mysql_query($sql, $link);
            
        if($result && mysql_affected_rows($link)) {
            return true;
        }else {
            return false;
        }
    }
    
    function destroy($sessionId) {
        global $link;
        
        $sessionId = mysql_escape_string($sessionId);
        
        $sql = "DELETE FROM session WHERE session_id='{$sessionId}';";
        $result = mysql_query($sql, $link);
        
        if($result) {
            return true;
        }else {
            return false;
        }
    }
    
    function gc($maxLifeTime) {
        return true;
    }