Interface

Interfaces have very specific rules in case two interfaces with the same member are implemented by a subtype:

• If both method definitions are abstract, then the method is abstract.
• If one method definition is abstract and the other concrete, then the concrete method is used.
• If both method definitions are concrete and the interfaces are implemented at the same level, then the method is abstract.
• If both method definitions are concrete and one of the interfaces is a sub-type of the other, then its method definition will be used.

In all of those cases, the rule is that a subtype overrides the parent types' definitions. Example: Given the interfaces A, B, C and D where A defines a method "a" to be abstract and B and C extending interface A with concrete implementations of "a", when D is a interface that implements both B and C, its version of "a" will be abstract, unless it defines its own implementation of "a", in which case that definition is used.

All interface types have a $class property, which contains their type, and a static isInterface method that can be used to test whether an object implements the interface.