PHP 5.5

I'm doing a bunch of passing around of objects with the assumption that they will all maintain their identities - that any changes made to their states from inside other objects' methods will continue to hold true afterwards. Am I assuming correctly?

I will give my basic structure here.

class builder {
  protected $foo_ids = array(); // set in construct
  protected $foo_collection;
  protected $bar_ids = array(); // set in construct
  protected $bar_collection;
  protected function initFoos() {
    $this->foo_collection = new FooCollection();
    foreach($this->food_ids as $id) {
      $this->foo_collection->addFoo(new foo($id));
    }
  }
  protected function initBars() {
    // same idea as initFoos
  }
  protected function wireFoosAndBars(fooCollection $foos, barCollection $bars) {
    // arguments are passed in using $this->foo_collection and $this->bar_collection
    foreach($foos as $foo_obj) { // (foo_collection implements IteratorAggregate)
      $bar_ids = $foo_obj->getAssociatedBarIds();
      if(!empty($bar_ids) ) {
        $bar_collection = new barCollection(); // sub-collection to be a component of each foo
        foreach($bar_ids as $bar_id) {
          $bar_collection->addBar(new bar($bar_id));
        }
        $foo_obj->addBarCollection($bar_collection);
        // now each foo_obj has a collection of bar objects, each of which is also in the main collection. Are they the same objects?
      }
    }
  }

}

What has me worried is that foreach supposedly works on a copy of its arrays. I want all the $foo and $bar objects to maintain their identities no matter which $collection object they become of a part of. Does that make sense?