In a recent post from Eric Adams, he shows how to use the SplObserver and SplSubject components of the Standard PHP Library (SPL) to make an implementation of the observer pattern for more reusable code.

As PHP applications grow into complex object-oriented systems, developers look to create centralized components to execute repetitive tasks. These include logging, emailing, redirects, and more. The Observer pattern is a commonly used design pattern to hook such components into an application during runtime, thereby making them reusable. Since PHP 5.1, there are two interfaces built into the Standard PHP Library (SPL) that can be implemented to use the Observer pattern in your application. They are SplSubject, and SplObserver.

He shows how to use them in an example of a custom error handler class that extends the SplSubject with its attach, detach and notify methods as well as logger and mailer classes that will use it to handle their exceptions.