On of the frameworks rising up through the ranks is Lithium (created by some of the original CakePHP developers). In his latest post Chris Hartjes looks at the framework and a particularly useful feature it includes - filters.

As far as I can tell, Lithium is trying it's hardest to be fast, lightweight, and use all the features that PHP 5.3 has to offer. [...] I want to talk about something that I feel is a very underrated part of Lithium, the ability to define filters. This is a concept that you find in Aspect-oriented programming, and one that I imagine most programmers have never considered. Here's why I think it's a big deal.

The filters in Lithium allow you to use code from other classes without having to actually extend them. As Nate Abele says in one of the comments on the post:

While not having to extend classes or dig inside core functionality is certainly pretty central, the way I've been explaining it is that it's important because classes no longer need to know anything about each other in order to be integrated.

You can find out more about the Lithium project (and RAD-Dev) on the project's wiki.