<?php
	class Car{
		public $engine;
	}
	class Engine{
		public $horsepower;
	}
	$myCar = new Car;
	$myCar->engine = ???;
	$myCar2 = new Engine;
	$mycar2->horsepower = 160;

	echo $myCar->engine . "<br>";
	echo $myCar2->horsepower . "<br>";
?>