A while ago I tweeted
ContainerAware is the new Singleton.
While many people agreed by retweeting and faving. I feel the need to elaborate some more on this statement and safe the explaination for the future.
TL;DR: No class of your application (except for factories) should know about the Dependency Injection Container (DIC).
The ContainerAware interface (actually ContainerAwareInterface, ContainerAware is a basic implementation of it) is part of the Symfony2 API, but a similar concept is known from many other frameworks and many applications rely on it. It defines only the one method setContainer(), which allows to inject the DIC into into an object so that it can directly retrieve services from it.
I wouldn't call it "the new Singleton", I'd call it "Service Location." If you use a dependency injection container inside your class to bring dependencies into the object, you are using Service Locator, not Dependency Injection. Solar used SL, but when we started Aura (a collection of truly decoupled library packages, each with no dependencies) we switched over to DI proper. Real DI has been a huge win.