James Morris has a new post to his blog talking about a way he's found to remove dependencies from parts of your code by using the Observer pattern (specifically with the SplObserver and SplSubject components of PHP's SPL libraries).
Working on a symfony app, you usually have a mix of domain objects that are used by symfony actions, interspersed with symfony specific code such as logging and sfContext type stuff. A common bad practice I see is symfony specific code peppered inside of domain objects that could be used elsewhere (such as inside of a Zend app or a script from the cli) but now can't as they're coupled to the symfony code. One way you can remove an unwanted dependency is to use the Observer pattern - the dependency is pushed from inside the subject object to the client code that initialises the subject.
He sets up a scenario where a symfony action hits a web service and there's a dependency on two other types of objects (a service and transport object). He starts with the code for this method and gradually refactors out those two dependencies, removing an sfContext call from inside the service client and adds attach/detach/notify methods to implement the SplObserver on the class.