按老师课程编写执行之后，报一个错误，百思不得其解，请求指点

附上authModel.class.php代码：

<?php

class authModel{

private $auth = '';

public function __construct(){

//判断当前是否已经登录

if($_SESSION['auth']&&(!empty($_SESSION['auth']))){

$this -> auth = $_SESSION['auth'];

}

}

//定义登录验证的方法

public function loginsubmit(){

if(empty($_POST['username'])||empty($_POST['password'])){

return false;

}

$username = daddslashes($_POST['username']);

$password = daddslashes($_POST['password']);

if($this->auth = $this->checkuser($username,$password)){

$_SESSION['auth'] = $this->auth;

return true;

}else{

return false;

}

}

public function checkuser($username,$password){

$auth = M('admin')->findOne_by_username($username);

if($auth&&$password == $auth['password']){

return $auth;

}else{

return false;

}

}

public function getAuth(){

return $this->auth;

}

}

?>