Object composition and reference

I'm compositing a class based on other classes. The component classes are (this is for a transportation application) fuel and vehicle. The container class is mode (as in mode of transit: rail, bus, etc.). There can be several vehicles and fuels for each mode (many to one). I want to be able to accomplish something like this: $bus = new mode(); $diesel = new fuel($bus); $ethanol = new fuel($bus); $class_a_bus = new vehicle($bus); $articulated_bus = new vehicle($bus); In other words, register the fuels and vehicles with the mode object in such a way that I don't have to make them members of said object; in further other words, instead of doing it something like: $bus = new mode(); $diesel = new fuel(); $bus->addFuel($diesel); Is there a way I can then reference the components of the mode object without having to know the names I picked for the variables; without having to do something like: $diesel = new fuel(); $bus->addFuel($diesel, 'diesel'); <-- a handle to reference the globally scoped variable One kludgy way would be to foreach() the $GLOBALS in a method of the mode class, checking for is_a('fuel'). One of my rules of thumb is to reduce use of the $GLOBALS. Would a more elegant solution be related to PHP's reference counting? I apologize if this is a confusing-as-hell post. I would be more than happy to clarify a point... Paul -- Paul Smith <paul@cnt.org> Center for Neighborhood Technology Chicago, IL USA