<?php

class ClassA {
    private $number;

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

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

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

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

        $sibling->test();
    }
}

class ClassC {

    public $a;

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

    public function test() {
        echo parent::getNumber();
    }
}

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

?>