Ian Barber has written up a look at dependency injection as a part of the Zend Framework's controller functionality for the Ibuildings blog.
Among the standard object oriented principles is favouring composition over inheritance, and there are plenty of design patterns that work along this line. However, one of the most useful day-to-day facets of the idea doesn't seem to get a lot of attention from PHP developers, namely dependency injection.
The general idea is, that if your class depends on some other object, that object should be passed in rather than generated internally or retrieved via a global variable or singleton.
He shares few ideas on how you can use this method in the controller of a Zend Framework including the use of the Zend Registry and an Action Helper. Code snips are provided for reach to show you how it'd be done.