PHP inheritance, parent functions using child variables

Question

While reviewing some PHP code I've discovered a strange thing. Here is the simple example illustration of it:

File A.php:

<?php
class A{
    public function methodA(){
        echo $this->B;
    }
}
?>

File B.php:

<?php
    class B extends A{
        public $B = "It's working!";
    }
?>

File test.php:

<?php
    require_once("A.php");
    require_once("B.php");
    $b = new B();
    $b->methodA();
?>

Running test.php prints out "It's working!", but question is why is it working? :) Is this a feature or a bug? Method methodA in class A can also call methods that are in class B which should not work in OOP.

Answer 1

You're only instantiating class B. Ignore A for the moment, and pretend that methodA() is part of class B.

When class B extends A, it gets all of A's functions. $this->B isn't evaluated until the code is running, not prior. Therefore no error occurs, and won't occur as $this->B exists in class B.