子 class 访问父 class 中的私有成员变量

Question

I try to create class User and Teacher extends from User class I have a problem with the Teacher class can access to private member variables in the parent class it's weird.

User Class

<?php

class User
{
    private $username;

    protected $password;

    public function login()
    {
        return 'login';
    }

    public function register()
    {
        return 'register';
    }
}

Teacher Class

<?php
class Teacher extends User
{
    public $id;

    public $name;

    public $description;

    public $email;

    public $phone;


    public function getUsername()
    {
        return $this->username;
    }


    public function getPassword()
    {
        return $this->password;
    }
}

PHP 7.2.14 (cli) (built: Jan 9 2019 22:23:26) ( ZTS MSVC15 (Visual C++ 2017) x64 ) Copyright (c) 1997-2018 The PHP Group Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies

Windows 10

Any logical reason?

Answer 1

$this->username does not call parent private member username , it creates new child object property username dynamically.

To keep username private in Teacher, you need to set field as protected. Protected fields means that is private for super class and also for children.

Answer 2

You can not access private properties or methods of class from outside of it.

Teacher::$username is an undefined property in this case (1. private property of parent is invisible for it's childs 2. it is not defined in itself), so whenever you call getUsername() a notice is thrown and depending on your error reporting config you'll see this on screen or not.

If you want to acces User::$username from Teacher class, you have to make it protected or public- what's the difference: check this for more informations https://www.php.net/manual/en/language.oop5.visibility.php