Accessing object's properties from another class - php

Question

I'm playing around with OOP in PHP and I've got the following code:
index.php:
<?php
include('user.class.php');
include('page.class.php');
$user = new User;
$page = new Page;
$page->print_username();
?>
user.class.php:
<?php
class User {
public function __construct() {
$this->username = "Anna";
}
}
?>
page.class.php:
<?php
class Page extends User {
public function __construct() {
}
public function print_username() {
echo $user->username;
}
}
?>
My problem occurs in the class "Page", in the print_username() function.
How do I access the $user object's properties within this class? I am, as you can see, defining the two objects in index.php.
Thanks in advance
/C

Answer 1

class User {
public $username = null;
public function __construct() {
$this->username = "Anna";
}
}
class Page extends User {
public function __construct() {
// if you define a function present in the parent also (even __construct())
// forward call to the parent (unless you have a VALID reason not to)
parent::__construct();
}
public function print_username() {
// use $this to access self and parent properties
// only parent's public and protected ones are accessible
echo $this->username;
}
}
$page = new Page;
$page->print_username();
$user should be $this.

Answer 2

class User {
public $username = null;
public function __construct() {
$this->username = "Anna";
}
}
class Page extends User {
public function print_username() {
echo $this->username; //get my name! ($this === me)
}
}

Answer 3

I see some confusion here:
You've caused your Page class to inherit from User. This means that the page itself has all the properties of the User class, and in fact, could be used in place of a User class. As the print_username() method is written in your code, it won't work - because it doesn't have a reference to a $user variable. You could change $user to $this to get a username in the print_username() method, and borrow from the parent class (User) to get the username property.
My thinking is though, that you didn't intend to do that. A Page is not a User, after all - they aren't related to each other. So what I'd do instead is remove extends User from the Page class. This will make a Page a Page, and a User a User.
But how is the Page going to print a username? Pages need to do that of course. What you could do instead is pass the $user object as a parameter to the Page's __construct() method, and then you can reference that value in your Page.
The first way of writing the code, using extends, involves inheritance. The second way of writing the code when passing in the user as a parameter involves composition. In a situation like this, with two separate ideas (Pages and Users), I would use composition to share and access object properties instead of inheritance.
I would do this instead:
<?php
class User {
public function __construct() {
$this->username = "Anna";
}
}
class Page {
private $user;
public function __construct($user) {
$this->user = $user;
}
public function print_username() {
echo $user->username;
}
}
$user = new User;
$page = new Page($user);
$page->print_username();
?>