How to mock a class created inside the tested class?

In the PHPUnit documentation, the section dealing with mocks explain how to attach a mock object inside the tested class :

...
$observer = $this->getMockBuilder('Observer')
                 ->setMethods(array('update'))
                 ->getMock();
...
$subject = new Subject('My subject');
$subject->attach($observer);
$subject->doSomething();

In my case, I want to test a Subject method. This method create many objects like this:

public function myMethod() {
    for($i = 0; $i < 10; $i++) {
        $this->collection []= new Observer(/* some data */);
    }
}

How can I mock all of the Observers?