Sometimes you see simple interfaces with a getter and a setter for a specific type of object. Some examples are league/event‘s EmitterAwareInterface and league/container‘s ImmutableContainerAwareInterface. Also Zend Framework 2 has a ton of *AwareInterfaces like (but not limited to) these:

These packages often come with a reusable trait that gives you pluggable implementations for these interfaces out of the box. For previous examples there are the EmitterAwareTrait and the ImmutableContainerAwareTrait.

I find these interfaces useful for setting up an IoC container to automatically “setter-inject” a dependency. With Laravel 5.0+’s container one can configure this:

With league/container‘s container one can use so called inflectors:

Now if you resolve any kind of object from the IoC container, it will have an EmitterInterface (or ImmutableContainer) object injected for you.