求大神纠错，多谢！Fatalerror:Calltoamemberfunctionquery()onanon-object

求大神纠错，谢谢！！Fatal error: Call to a member function query() on a non-object
如题，出现错误。Fatal error: Call to a member function query() on a non-object in D:\WWW\2shou\class\Bulletin.php on line 47
代码如下：
//本类用于保存对表Bulletin的数据库访问操作
//表的每个字段对应类的一个成员变量
Class Bulletin
{
public $Id; // 记录编号
public $Title; // 公告标题
public $Content; // 公告内容
public $PostTime; // 发布日期
public $Poster; // 发布人
var $conn;

function __construct() {
// 连接数据库
$this->conn = mysqli_connect("localhost", "root", "pass", "2shou");
mysqli_query($this->conn, "SET NAMES gbk");
}

function __destruct() {
// 关闭连接
mysqli_close($this->conn);
}

// 获取公告信息
function GetBulletinInfo($bid) {
//设置查询的SELECT语句
$sql = "SELECT * FROM Bulletin WHERE Id='" . $bid . "'";
// 打开记录集
$results = $this->conn->query($sql);
// 读取公告数据
if($row = $results->fetch_row()) {
$this->Id = $bid;
$this->Title = $row[1];
$this->Content = $row[2];
$this->PostTime = $row[3];
$this->Poster = $row[4];
}
else {
$this->Id=0;
}
}

// 获取所有公告信息，返回结果集
function GetBulletinlist() {
//设置查询的SELECT语句
$sql = "SELECT * FROM Bulletin ORDER BY PostTime DESC";
$results = $this->conn->query($sql);
return $results;
}

// 获取所有公告信息，返回结果集
function GetRecentBulletinlist() {
//设置查询的SELECT语句
$sql = "SELECT * FROM Bulletin WHERE DateDiff(day, getdate(), Posttime)<=7";
$results = $this->conn->query($sql);
return $results;
}

// 添加公告信息
function insert() {
$sql = "INSERT INTO Bulletin (Title, Content, PostTime, Poster) VALUES ('" . $this->Title . "','" . $this->Content . "','" . $this->PostTime . "','" . $this->Poster . "')";
// 执行SQL语句
$this->conn->query($sql);
}

// 修改公告信息
function update($bid) {
$sql = "UPDATE Bulletin SET Title='" . $this->Title . "', Content='" . $this->Content . "', PostTime='" . $this->PostTime . "', Poster='" . $this->Poster . "' WHERE Id=" . $bid;
// 执行SQL语句
$this->conn->query($sql);
}

// 批量删除公告信息
function delete($bid) {
$sql = "DELETE FROM Bulletin WHERE Id IN (" . $bid . ")";
// 执行SQL语句
$this->conn->query($sql);
}
}
?>

------解决思路----------------------
所以构造函数应写作

function __construct() {
  $this->conn = new mysqli("localhost", "root", "pass", "2shou");
  $this->conn->set_charset('gbk');
}