Description:
------------
Currently class type can be specified as function argument type, but it is frequently required to pass null instead of object reference when there is a default argument value set and it is null. In the following example constructor's $parent argument can be null for the top-level objects, but current PHP version doesn't allow it to be null forcing to remove type declaration that is very undesirable:

  class ExElement extends Exception { };

  class Element {
    protected /* Document */ $document; // each element references document for fast access
    protected /* Element */ $parent;

    function __construct(Document $document, Element $parent = null) /* throws ExElement */ {
      $this->document = $document; // is still checked by PHP to be valid Document object reference
      if ($parent && ($parent->getDocument() != $document))
        throw new ExElement("Parent's document doesn't match Element constructor's argument", 1);
      $this->parent = $parent;
    }

    function getDocument() {
      return $this->document;
    }
  }