show only parent class properties in child class using $this in parent class

I have the following two classes. Class BMW extends class Car.

class Car{

    public $doors;
    public $wheels;
    public $color;
    public $size;

    public function print_this(){
        print_r($this);
    }

}

class BMW extends Car{
    public $company;
    public $modal;

    public function __construct(){
        print_r(parent::print_this());
    }
}

$bmw = new BMW();
$bmw->print_this();

In above code when I access parent class method from constructor using parent::print_this() and inside print_this() method I have print_r($this) which prints all properties(parent and child class properties) Now what I want print_r(parent::print_this()); should output only parent class properties in child class? Can anyone help me on this?