An interface is a programming structure that allows the computer to enforce certain properties on an object. In object oriented programming, an interface generally defines the set of methods that an instance of a class that has that interface could respond to. It is actually a concept of abstraction and encapsulation.
We already discussed about the basic concepts and terms in Domain Driven Design (DDD), check out the Introduction post if you missed it. Also we explained which Layers compose the DDD architecture. Now let's see the artifacts we use to unify these concepts and build our application (with PHP example).