PHP 5 Object Inheritance

Hi, I've got a collection of Element classes (about 8 different ones). They are all subclasses of a single parent element. I'm trying to extend their functionality (both the individual classes, and the parent class they inherit). I can extend each Element subclass with it's new specific functionality, but I would also like to add specific functionality to all of the subclasses. Extending the parent element with the new functionality would *seem* to be the way to go, but I can't make it work None of the Subclasses inherit from the extended superclass. Any thoughts? I'd appreciate any ideas. Thanks, Chris