I think I've a problem in understanding how OOP works. I already changed the code that it works, but it isn't the propper way I think. Following scenario (No, I'm not creating a userlogin by myself, its really just for local dev. to understand OOP better):

我想我在理解OOP是如何工作的方面有问题。我已经改变了它的工作代码，但它不是我认为的正确方式。下面的场景（不，我不是自己创建一个用户登录，它真的只是为本地开发。更好地理解OOP）：

I've a database.php file:

我有一个数据库.php文件：

class Database {

    /* Properties */
    private $conn;
    private $dsn = 'mysql:dbname=test;host=127.0.0.1';
    private $user = 'root';
    private $password = '';

    /* Creates database connection */
    public function __construct() {
        try {
            $this->conn = new PDO($this->dsn, $this->user, $this->password);
        } catch (PDOException $e) {
            print "Error!: " . $e->getMessage() . "";
            die();
        }
        return $this->conn;
    }
}

So in this class I'm creating a database connection and I return the connection (object?)

在这个类中，我创建了一个数据库连接，并返回连接（对象？）

Then I have a second class, the famous User class (actually I'm not using autoload, but I know about it):

然后我还有第二个类，著名的User类(实际上我没有使用autoload，但我知道它)：

include "database.php";

class User {
    /* Properties */
    private $conn;

    /* Get database access */
    public function __construct() {
        $this->conn = new Database();
    }

    /* Login a user */
    public function login() {
        $stmt = $this->conn->prepare("SELECT username, usermail FROM user");
        if($stmt->execute()) {
            while($rows = $stmt->fetch()) {
                $fetch[] = $rows;
            }
            return $fetch;
        }
        else {
            return false;
        }
    }
}

So thatare my two classes. Nothing big, as you see. Now, don't get confued about the function name login - Actually I just try to select some usernames and usermails from database and displaying them. I try to achieve this by:

这就是我的两节课。如你所见，没什么大不了的。现在，不要对函数名LOGIN感到困惑--实际上，我只是尝试从数据库中选择一些用户名和用户邮件并显示它们。我试图通过以下方式来实现这一点：

$user = new User();
$list = $user->login();

foreach($list as $test) {
    echo $test["username"];
}

And here comes the problem. When I execute this code, I get the following error message:

现在问题来了。当我执行此代码时，我收到以下错误消息：

Uncaught Error: Call to undefined method Database::prepare()

And I'm not sure that I really understand what causes this error.

我不确定我是否真的理解是什么导致了这个错误。

The code works well when I change the following things:

当我更改以下内容时，代码运行良好：

Change $conn in database.php to public instead of private (I think thats bad...? But when its private, I can only execute querys inside of the Database class, I'm right? So should I put all these querys in the Database class? I think that's bad, because in a big project it will get become really big..)

将datase.php中的$conn更改为PUBLIC而不是PRIVATE(我认为这很糟糕...？但是当它是私有的时，我只能在数据库类中执行查询，对吗？那么，我应该将所有这些查询放在数据库类中吗？我认为这很糟糕，因为在一个大项目中，它会变得非常大。)

And the second change I've to do is:
Change $this->conn->prepare to $this->conn->conn->prepare in the user.php file. And here I've really no Idea why.

我要做的第二个更改是：在user.php文件中将$This->Conn->Prepare更改为$This->Conn->Conn->Prepare。在这里，我真的不知道为什么。

I mean, in the constructor of the user.php I've a $this->conn = new Database() and since new Database will return me the connection object from DB class, I really don't know why there have to be a second conn->

我的意思是，在user.php的构造函数中，我有一个$This->conn=new Database()，既然new Database将从DB类返回给我Connection对象，我真的不知道为什么一定要有第二个conn->

• Do not create classes such as your Database class as it's rather useless. It would make sense to create a database wrapper if it adds some extra functionality to PDO. But given its current code, better to use vanilla PDO instead.


• Create a single $db instance from either vanilla PDO or your database class.


• Pass it as a constructor parameter into every class that needs a database connection

database.php:

数据库.php：

<?php
$host = '127.0.0.1';
$db   = 'test';
$user = 'root';
$pass = '';
$charset = 'utf8';

$dsn = "mysql:host=$host;dbname=$db;charset=$charset";
$opt = [
    \PDO::ATTR_ERRMODE            => \PDO::ERRMODE_EXCEPTION,
    \PDO::ATTR_DEFAULT_FETCH_MODE => \PDO::FETCH_ASSOC,
    \PDO::ATTR_EMULATE_PREPARES   => false,
];
$pdo = new \PDO($dsn, $user, $pass, $opt);

user.php

User.php

<?php
class User {
    /* Properties */
    private $conn;

    /* Get database access */
    public function __construct(\PDO $pdo) {
        $this->conn = $pdo;
    }

    /* List all users */
    public function getUsers() {
        return $this->conn->query("SELECT username, usermail FROM user")->fetchAll();
    }
}

app.php

App.php

include 'database.php';
$user = new User($pdo);
$list = $user->getUsers();

foreach($list as $test) {
    echo $test["username"],"\n";
}

output:

输出：

username_foo
username_bar
username_baz

Check out my (The only proper) PDO tutorial for more PDO details.

查看我的(唯一合适的)PDO教程，了解更多的PDO细节。