Zend now keeps track of all references to this resource. As soon as all references to the resource are lost, the destructor that you previously registered for this resource is called. The nice thing about this setup is that you don't have to worry about memory leakages introduced by allocations in your module just register all memory allocations that your calling script will refer to as resources. As soon as the script decides it doesn't need them anymore, Zend will find out and tell you.