In this new tutorial on PHPMaster.com, Alejandro Gervasio looks at the Inversion of Control methodology and how it's more than just an abstract reference to dependency injection.

Traditionally, application components have been designed to operate on and control the execution environment, an approach that delivers well to some extent. [...] Instead of making the module completely responsible for logging data to multiple endpoints, we can transfer the responsibility straight to the external environment. [...] Not surprisingly, the process of inverting these responsibilities between components and the environment is formally known as Inversion of Control (or in a more relaxed jargon, The Hollywood Principle), and its implementation can be a real boost when it comes to developing extensible, highly-decoupled program modules.

He uses a set of domain objects (Posts and Comments in a typical blog structure) and the Observer pattern to show how mixed up things might get if the application isn't carefully coded. He takes this and updates it to include a "comment notification service" that implements the SplObserver and is attached to the post to be executed on an event (in this case, the setting of a new comment).