Ryan Gantt has a new tutorial posted to his blog today looking at one of the features in the upcoming PHP 5.4.x releases - traits. Specifically he looks at the horizontal reusabillity they allow for in your applications.
The ability for a class to inherit from multiple parents is maligned by many, but can be a good thing in some situations. For those working in PHP, multiple inheritance has never been an option; classes are limited to one parent, though they can implement many other datatypes through the use of interfaces. Interfaces can lead to code duplication in improperly-factored inheritance hierarchies. Even in well-architected hierarchies, multiple classes that implement similar methods can contain a lot of overlap.
He starts with a definition of what traits are and where their real usefulness is (as well as what should be the difference between a class and a trait). He gives an example of a typical hierarchy where two classes extend a parent but then they both need the same functionality. Code duplication's not a possibility and inheritance make run into exposure issues. Traits come to the rescue by dropping in just the feature you need when you need it. His example code shows adding some logging to a simple class via a "Logging" trait and a "Singleton trait" example.