In his latest tutorial, Alexander Netkachev shows how to, with some of the simple PHP functions, create an event system for your script, complete with callbacks.
The way how events are raised and how listeners are attached on the events is a part of a core in many modern applications. It plays an important role in some enterprise design patterns (MVC, for example).
He starts with the basics of event handling - some of the terms and descriptions of basic functionality that any good event handler would have. He describes the most common setup of an event-interaction relationship. Then, it's on to the code, showing first three different ways to call functions (by name, by variable, and by callback).
He finishes it off with a functional example that responds to a a call to fireEvent (five times) and handles each by calling the function in the callback information (myFunction).