php怎么把session保存到MySql数据库中

class CustomSession{
 private static $db_host="localhost";
 private static $db_user="root";
 private static $db_password="";
 private static $database="session";

 private $conn;

 public static function getInstance(){
  static $instance=null;
  if($instance==null){
   $instance=new CustomSession();
  }

  return $instance;
 }

 public function __construct(){
  session_set_save_handler(
  array($this,"open"),
  array($this,"close"),
  array($this,"read"),
  array($this,"write"),
  array($this,"destroy"),
  array($this,"gc")
  );
 }

 public function __destruct(){
  session_write_close();
 }

 public function open(){
  $this->conn=mysql_connect(CustomSession::$db_host,CustomSession::$db_user,CustomSession::$db_password);
  mysql_select_db(CustomSession::$database,$this->conn);
 }

 public function close(){

  mysql_close($this->conn);
 }

 public function read($id){
  $escaped_id=mysql_escape_string($id);
  $res=$this->query("select * from `session` where `sessionid`='$escaped_id'");
  if($row=mysql_fetch_assoc($res)){
   $this->query("update `session` set `timetamp`=UTC_TIMESTAMP() where `sessionid`='$escaped_id'");
   return $row['data'];
  }
  return "";
 }

 public function write($id,$data){
  $query="replace into `session` (`sessionid`,`data`,`ip`,`timestamp`) values ('%s','%s','%s',UNIX_TIMESTAMP(UTC_TIMESTAMP()))";
  $this->query(sprintf($query,mysql_escape_string($id),mysql_escape_string($data),$_SERVER["REMOTE_ADDR"]));//www.111com.net
 }

 public function destroy($id){
  $escaped_id=mysql_escape_string($id);
  $res=$this->query("delete from `session` where `id`='$escaped_id'");
  return (mysql_affected_rows($res)==1);
 }

 public function gc($lifetime){
  $this->query("delete from `session` where UNIX_TIMESTAMP(UTC_TIMESTAMP())-`timestamp` > $lifetime");
 }

 public function query($query){
  $res=mysql_query($query,$this->conn);
  return $res;
 }
}

?>

include('./CustomSession.class.php');

CustomSession::getInstance();

session_start();

$_SESSION['username']='feng';

print_r($_SESSION);

?>