Metadata, active properties, garbage collection

A few questions on Zend internals: Is it possible to add (opaque, as far as the PHP script will be concerned) data to object of internal class from C? Preferably, this data would not even be visible to the script. I see the class declaration struct has functions that seem to be related to getting and setting properties. How would I use these to create 'active' attributes? When the refcount of a value drops to 0, when is it garbage collected? Can I hook into that (from C) to clear up related resources? Emile