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.
Previously, we explored the fundamental concepts and terminology utilized in Domain Driven Design (DDD), as well as the layers that make up its architecture. In this section, we will examine the artifacts employed to integrate these concepts and construct our application, using PHP as an example.
Enumerations, or Enums allow a developer to define a custom type that is limited to one of a discrete number of possible values. That can be especially helpful when defining a domain model, as it enables making invalid states unrepresentable. In other words, enums are a restricting layer on top of classes and class constants, intended to provide a way to define a closed set of possible values for a type.
