PHP Helper Class Puzzler... O_o

<?php

class Main {
        var $x = 3;
        function a(){
                $this->Child = new Child(&$this);
        }
}

class Child {
        function Child(&$m){

                print $m->x;
        }
}

$Main = new Main();

$Main->a();


?>

$this->Child = new Child(&$this);

class Main {
	protected $number = 3;
	
	public function create($class) {
		if (class_exists($class)) {
			$this->$class = new $class($this);
		}
		return $this->$class;
	}
	
	public function getNumber() {
		return $this->number;
	}
}

class Child {
	function __construct(Main &$main) {
		$this->main =& $main;
		print $this->main->getNumber();
	}
}

$main = new Main();
$main->create('Child');

class SOME_CONFIG {
   var $x = 'something';
}

$config['x'] = 'something';