警告：mysql_num_rows（）需要参数1

Question

我试图让我的登录主页工作，但我＆＃39;总是得到以下错误： ＆＃34;警告：mysql_num_rows（）期望参数1是资源＆＃34; 在getData方法中，我尝试返回true作为结果以在loginController.php中启动会话

    private function getData(){

          $query ="SELECT * FROM 'users' WHERE 'username' = '$this->username'
                                    AND 'password' = '$this->password'";
           $sql = mysql_query($query) ; 

           if(mysql_num_rows($sql)>1){
            return TURE;
           }else{
               throw new Exception("Username or password is invalid, please try again!");
                } 
        }

<?php


     if (isset($_POST['submit']) AND $_POST['submit']== 'Login') {
          $username = $_POST['username'];
          $password = $_POST['password'];

                   try{
                   include "./models/login.php";
                   $login = new Login($username, $password);

                   if ($login== TRUE) {
                     session_start();
                     $_SESSION['username'] = $username;
                     header('Location: index.php');
                   }


                   }catch(Exception $exc ){
                   echo $exc->getMessage();
                   }
             }


?>