PHP中SESSION自定义会话管理器

<?php
class CustomSession implements SessionHandlerInterface{
      private $link;
      private $lifetime;
      public function open($savePath,$session_name){
          $this->lifetime=get_cfg_var('session.gc_maxlifetime');
          $this->link=mysqli_connect('localhost','root','root','session_test');
          mysqli_query($this->link,"SET names UTF8");
          if($this->link){
              return true;
          }
          return false;
      }
      public function close(){
              mysqli_close($this->link);
              return true;
      }
      public function read($session_id){
              $sql="SELECT *from sessions where session_id='{$session_id}'
              and session_expires >".time();
              $result=mysqli_query($this->link,$sql);
              if(mysqli_num_rows($result)){
                  return mysqli_fetch_array($result)['session_data'];
              }
              return "";
      }
      public function write($session_id,$session_data){
              $newExp=time()+$this->lifetime;
              //首先查询是否存在指定的session_id,如果存在相当于更新数据，否则是第一次，则写入数据
              $sql="SELECT * from sessions where session_id={'$session_id'}";
              $result=mysqli_query($this->link,$sql);
              if(mysqli_num_rows($result)==1){
                  $sql="UPDATE sessions set session_expires='{$newExp}',session_data='{$session_data}' 
                  where  session_id='{$session_id}' ";
              }else{
                  $sql="INSERT into sessions values('{$session_id}','$session_data','{$newExp}')";
              }
              mysqli_query($this->link,$sql);
              return mysqli_affected_rows($this->link)==1;

      }        
      public function destroy($session_id){
              $sql="DELETE from sessions where session_id='{$session_id}'";
              mysqli_query($this->link,$sql);
              return mysqli_affected_rows($this->link)==1;
      }
      public function gc($maxlifetime){
              $sql="DELETE from sessions where session_expires<".time();
              mysqli_query($this->link,$sql);
              if(mysqli_affected_rows($this->link)>0){
                  return true;
              }
              return false;
      }
}

<?php 
    header("Content-type:text/html;charset=utf-8");
    require_once 'customSession.php';
    $customSession=new CustomSession();
    ini_set('session.save_handler', 'user');
    session_set_save_handler($customSession,true);
    session_start();
    $_SESSION['name']='admin';
    $_SESSION['pwd']='123456';

//销毁会话，从数据库中删除
<?php 
    require_once 'customSession.php';
    $customSession=new CustomSession();
    ini_set('session.save_handler', 'user');
    session_set_save_handler($customSession,true);
    session_start();
    session_destroy();