In his most recent blog post Maarten Balliauw proposes an idea of something to be included into PHP that .NET developers are used to as commonplace in their language - extension methods.
For the PHP-related audience, this is probably something new. Let's start with the official definition for extension methods: Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.
He gives a code example of the concept, showing how you could add this sort of functionality to a PHP class with the help of the __call magic method and some of the Reflection features. The real usefulness, he notes, would be if the idea made it into the core rather than having to do it with the hack he's shown.