On PHPMaster.com today they've posted an introduction to the Law of Demeter, an idea that promotes loose coupling between objects that each do their own work. This makes it simpler to make pieces interchangeable and more able to focus on their job.
With so many heuristics stating how and why software systems should cling to a specific approach, it's pretty disappointing not seeing a broader implementation of them in the world of PHP. For example, the Law of Demeter is probably one of the most underrated in the language's realm. Effectively, the law's "talk to your closest friends" mantra still seems to be in a pretty immature state in PHP, something that contributes to rot in the overall quality of several object-oriented code bases.
They look at how keeping the functionality of each object focused and not "knowing too much" is a good thing and include some examples of working with a service locator, serializer and file storage classes.