Gonzalo Ayuso has a new post that responds to the idea that "singletons are a problem when testing" your applications with something like PHPUnit.
Maybe this pattern is not as useful as it is in J2EE world. With PHP everything dies within each request, so we cannot persist our instances between requests (without any persistent mechanism such as databases, memcached or external servers). But at least in PHP we can share the same instance, with this pattern, in our script.
He illustrates a bad side effect of this sharing of resources with a simple unit test that increments a counter in a class. He notes that, because the script shares the object, you can't reliably know the state of it as you don't know what's happened before your use. He recommends two things to help the situation - either not use them at all or destroy the instance each time after using it (counterproductive to using a Singleton, obviously).