致命错误：在null上调用成员函数query（）?mysql

我不确定这里出了什么问题。我只是在线上学习教程，并且弹出了这些错误。

我收到以下错误

错误

Notice: Undefined variable: db in C:\xampp\htdocs\wisconsindairyfarmers\admin\login.php on line 7

Fatal error: Call to a member function query() on null in C:\xampp\htdocs\wisconsindairyfarmers\admin\login.php on line 7 码

query("SELECT COUNT(UserId) FROM users WHERE UserName = '$username'"); if($result->num_rows){ return (mysqli_result($query, 0) == 1) ? true : false; }} if(empty($_POST) === false){ $username = $_POST['username']; $password = $_POST['password']; if(empty($username) === true || empty($password) === true){ echo 'You need to enter a username and password'; } else if(user_exists($username) === false) { echo 'We can\'t find that username.'; } } ?>