How to Restrict Object Instantiation to Other Classes?

[tomcatexodus]

I'm looking for a way to restrict instantiation.

If I have a class named Machine, and a class named Widget, how can I prevent the Widget class from being instantiated unless it is performed by a method in the Machine class?

I know that nested classes are not valid, but this code exemplifies the functionality I'm looking to achieve:

class Machine{

    private class Widget{
        //Widget properties and methods
    }

    public function makeWidget(){
        $newWidget = new Widget();
        return $newWidget;
    }

    //Machine properties and methods

}

$myMachine = new Machine();
$myWidget = $myMachine->makeWidget(); //would work fine

$myOtherWidget = new Widget(); //would fail 

How can I achieve this?