In his latest post today, Ivo Jansch talks about a concpt from Ruby that he likes and wanted to try to implment in PHP - mixins.
Mixins are a way of 'mixing in' functionality of other classes. A kind of 'multiple inheritance' like approach, only without actual inheritance. It is similar to interfaces, but interfaces only tell you that an object must implement certain methods, whereas mixins also provide an implementation.
To help clear it up a bit he links to two articles on the topic. He gives some examples, though, of how it all works by creating a mixin class (Alertable) and the wy to apply it (in the Hello class). The key between the two is the Object class he's created. It allows the two other classes to work with each other without the need for cumbersome includes or passing around objects. All methods are magically available to the Hello object.
Of course, it's not a perfect implementation because PHP just can't do some things, but he includes a few of these things to watch out for in using this method.