How to use child class to change parents property - php

Question

I'm trying to change a property in my parent class with my child class but I'm not getting the result I'm expecting.
I've done some research (like Change parent variable from child class), but I can't seem to find the problem.
class A {
public $msg;
public function __construct() {
$this->msg = 'foo';
}
public function setMessage($string) {
$this->msg = $string;
}
public function getMessage() {
var_dump($this->msg); // For demo purposes
}
public function triggerB() {
$b = new B;
}
}
class B extends A {
public function __construct() {
parent::setMessage('bar');
}
}
$a = new A;
$a->getMessage();
$a->triggerB();
$a->getMessage();
The output I get is "foo" twice and I expect it to be "foo" "bar".
Could anyone explain me what i'm doing wrong and how I can fix this?
In my actual code I want the child-class to validate some $_POST values, and return the outcome to the Main-class. The parent uses the child to validate.

Answer 1

You are having your object A and creating an instance of it and storing it in the variable $a, in the global scope. And then you are creating another instance of your class B and storing it in a variable $b which is in the scope of the method triggerB().
You can only change the properties of the parent class A if you pass an argument to your another class B.
So something like this should suffice:
<?php declare(strict_types = 1);
class A {
public $msg;
public function __construct() {
$this->msg = 'foo';
}
public function setMessage(string $string) {
$this->msg = $string;
}
public function getMessage() {
var_dump($this->msg); // For demo purposes
}
public function triggerB() {
(new B($this));
}
}
class B {
public function __construct(A $a) {
$a->msg = "bar";
}
}
$a = new A;
$a->getMessage();
$a->triggerB();
$a->getMessage();
This approach is better suited to readability and better dependency management.
Another approach:
<?php declare(strict_types = 1);
class A {
public $msg;
public function __construct() {
$this->msg = 'foo';
}
public function setMessage(string $string) {
$this->msg = $string;
}
public function getMessage() {
var_dump($this->msg); // For demo purposes
}
public function triggerB() {
$this->msg = 'bar';
}
}
class B {
public function __construct(A $a) {
$a->msg = "bar";
}
}
$a = new A;
$a->getMessage();
$a->triggerB();
$a->getMessage();
This is performance wise better, but if you are going to be doing something complex, the first method is better.
Note: The above code is for PHP7.

Answer 2

Your triggerB() method does not actually do anything:
public function triggerB() {
$b = new B;
}
You are creating a new object and assign that to the $b variable. As soon as the method finishes, the $b variable / object ceases to exist.
Also note that the $b variable in your method is is no way related to the $a variable in the global scope so setting any of its properties has no influence on $a.