The PHP interpreter currently uses a simple reference counting mechanism to keep track of variables in memory, freeing them when the number of references to them reaches zero. Although this system is simple and effective, its major weakness is its inability to handle cycles of objects referencing each other.

Implementing an optional cycle collector similar to one described by Bacon and Rajan in "Concurrent Cycle Collection in Reference Counted Systems" into the PHP interpreter will address this weakness and is the goal of this project.