为什么我的班级找不到其他班级的变量

Question

我是 OOP 的新手，为什么我在其他文件中的类在我包含的其他类中找不到变量..？我尝试发送变量 $db 但它给了我错误。第一课<?php//making Database connectionclass DBconnection{    public function __construct()    {        DEFINE("DB_USER", "root"); // username database        DEFINE("DB_PASS", ""); // password database        try {            $db = new PDO("mysql:host=localhost;dbname=php_opdracht2", DB_USER, DB_PASS); //host & database name            $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);            echo "Connected successfully";        } catch (PDOException $e) {            echo $e->getMessage();        }    }}//running the class DBconnection$obj = new DBconnection();第二类（另一个 .php 文件）<?php//getting the database connectionrequire_once('DBconnection.php');//getting the data from the databaseclass Controller{    public function processing()    {        try {            $query   = "SELECT * FROM information";            $sth     = $db->query($query); //why does it not find $db..        } catch (PDOException $e) {            echo $e->getMessage();        }    }}$obj3 = new Controller();$obj3->processing();

Answer 1

在您的 DBconnection 类中，声明一个公共变量来保存数据库连接

<?php

//making Database connection

class DBconnection {

    public $db;

    public function __construct() {

        DEFINE("DB_USER", "root"); // username database

        DEFINE("DB_PASS", ""); // password database

        try {

            $this->db = new PDO("mysql:host=localhost;dbname=php_opdracht2", DB_USER, DB_PASS); //host & database name

            $this->db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

            echo "Connected successfully";

        } catch (PDOException $e) {

            echo $e->getMessage();

        }

    }

}

然后在另一个类中，访问它如下

<?php

//getting the database connection

require_once('DBconnection.php');

//getting the data from the database

class Controller {

    public function processing() {

        try {

            $connection = new DBconnection(); #call db class

            $query = "SELECT * FROM information";

            $sth = $connection->db->query($query); //access the db variable created in db class

        } catch (PDOException $e) {

            echo $e->getMessage();

        }

    }

}