php - 在构造函数中定义的未定义变量

Question

嗨，我的代码似乎出现错误，似乎说我没有初始化我的变量，但是在构造函数中初始化了 veriabile：

Notice: Undefined variable: conn in D:\Program Files\xampp\htdocs\Tutorials\Login\classes\mysql.php on line 18

Fatal error: Cannot access empty property in D:\Program Files\xampp\htdocs\Tutorials\Login\classes\mysql.php on line 18

这是我的代码：

<?php
require_once('includes/constants.php');
    class Mysql{

        private $conn;

        function __construct() {
            $this->conn = new mysqli(DB_SERVER , DB_USER , DB_PASSWORD , DB_NAME) or 
                   die('There was a problem connecting to the database');
        }
        function verify_Username_and_Pass($un , $pwd){

            $query = "SELECT * 
                      FROM users
                      WHERE username = ? AND password= ?
                      LIMIT 1";

            if($stmt = $this->$conn->prepare($query)){ //this is where the error points
                $stmt->bind_param('ss', $un, $pwd);
                $stmt->execute();

                if($stmt->fetch()){
                    $stmt->close;
                    return true;
                }
            }   
        }
    }
?>

在第一个 if 语句时抛出错误。我该如何解决？

Answer 1

代替

$this->$conn

和

$this->conn

出现错误时上线。

Answer 2

$this->$conn

这是错误的，应该是

$this->conn