OOP PHP

<?php

class ClassA {
    private $number;

    public function setNumber($number) {
        $this->number = $number;
    }

    public function getNumber() {
        return 'The number is: ' . $this->number;
    }
}

class ClassB extends ClassA {
    public function test() {
        $sibling = new ClassC($this);

        parent::setNumber(12);
        echo parent::getNumber() . PHP_EOL;
        parent::setNumber(17);

        $sibling->test();
    }
}

class ClassC {

    public $b;

    public function __construct($b) {
        $this->b = $b;
    }

    public function test() {
        echo $this->b->getNumber();
    }
}

$obj = new ClassB();
$obj->test();

?>