Recursive Iteration over a collection of objects

Hi I am getting myself quite confused while trying to use SPL to recursively iterate through a collection of objects, and any help would be greatly appreciated! My collection of objects is contained in a class (which I frequently use to iterate through objects stored in an array in the class), and I think my hasChildren() and getChildren() methods are working ok, but I am unable to make it recursive. I am particularly confused by where I have to use RecursiveIteratorIterator (do I have to at all?), and the getIterator() method (again, do I need it and why?). Am I going about this totally the wrong way to start with? I have tried to summarise my code below. Sorry for the long post, and thanks again. David ====== Script... ========== $categories = new RecursiveIteratorIterator(new Categories); $categories->loadAll(); foreach ($categories as $category) {

    echo 'ID: ' . $category->getID();
    echo 'Level: ' . $category->getLevel();

} ======= classes... ========= abstract class ObjCollection implements RecursiveIterator {

    // class stores objects in an array
    public $objs = array();
    public function current();
    public function hasChildren();
    // etc etc

} class Categories extends ObjCollection {

    // concrete class which loads the Category objects into
    // the ObjCollection array
    public function loadAll() {// ...load Category classes
                               // into parent:: !}

} class Category {

    private $id;
    private $level
    public function getID() {return $this->id;}
    public function getLevel() {return $this->level;}

}