I realize the knee-jerk response to this question is that "you dont.", but hear me out.
Basically I am running on an active-record system on a SQL, and in order to prevent duplicate objects for the same database row I keep an 'array' in the factory with each currently loaded object (using an autoincrement 'id' as the key).
The problem is that when I try to process 90,000+ rows through this system on the odd occasion, PHP hits memory issues. This would very easily be solved by running a garbage collect every few hundred rows, but unfortunately since the factory stores a copy of each object - PHP's garbage collection won't free any of these nodes.
The only solution I can think of, is to check if the reference count of the objects stored in the factory is equal to one (i.e. nothing is referencing that class), and if so free them. This would solve my issue, however PHP doesn't have a reference count method? (besides debug_zval_dump, but thats barely usable).
It seems like the best answer was still getting the reference count, although debug_zval_dump and ob_start was too ugly a hack to include in my application.
Instead I coded up a simple PHP module with a refcount() function, available at: http://github.com/qix/php_refcount