<?
class A
{
    var $var_a;

    function A()
    {
        $this->set_a("old");
    }

    function set_a($a) { $this->var_a=$a; }
}

class B
{
    var $obj_a;

    function B()
    {
        $this->obj_a=new A;
    }
}

$my=new B;
var_dump($my);

$my->obj_a->set_a("new");
var_dump($my);

?>

I create new object from class B ($my)

variable $obj_a in object $my IS OBJECT from class A.

var_dump($my) return 

object(b)(1) { ["obj_a"]=> object(a)(1) { ["var_a"]=> string(3) "old" } } 

It's ok.

After that I call a method from class A

$my->obj_a->set_a("new");

This method change variable $var_a, but after that variable $obj_a IS REFERENCE to object A

var_dump($my) return :

object(b)(1) { ["obj_a"]=> &object(a)(1) { ["var_a"]=> string(3) "new" } } 

$obj_a was object, but now $obj_a is reference to object.