Paul Jones presents a problem he was having in a new post to his blog - the issue was with memory leaks in objects in a PHP5 application he was working on.
One of the nice things about using a scripting language is that it automates garbage collection for you. You don't have to worry about releasing memory when you're done with your variables. [...] But there is at least one circumstance in which PHP will not free memory for you when you call unset(). Cf. http://bugs.php.net/bug.php?id=33595.
The problem is that, when trying to unset the parent object in a parent/child relationship, the memory space for the parent reference in the child isn't removed. The problem, left unchecked can cause a script to go over the memory limit pretty easily (especially with larger objects).
He suggests two solutions - one that the user can control (calling a destructor before unsetting the object) or a PHP internals method that involves fixing issues with the reference counts created with the objects.