Is there any way to force PHP to extend an existing/particular (and for that matter, already instantiated) instance of an object?

This imaginary code should explain what I am wondering:

class Base
{
     public $var;

     function __construct($var){
         $this->var = $name;
     }

}

class Extender extends Base { 

    function __construct($parent)
    {
         parent = $parent;  
    }

}

$base = new Base('yay!');

$extender = new Extender($base);

echo 'Extended base var value: '.$extender->var.'<br/>';

Output (would be):
Extended base var value: yay!

To be clear, I am wanting to instantiate an object that extends a PARTICULAR INSTANCE of another object, one that has already been instantiated.

I am aware that I can pass a reference to an object to another object (via it's constructor function) and then add it as a property of the receiving object, I'm wondering if there is a real way to do this?