Cheat Sheet on Classes, Traits and Interfaces on PHP/Hack

Haven't coded any PHP/Hack for a while, just recall my memory when I need to use Classes, Traits and Interfaces on PHP/Hack. :)

Classes

• It is the blueprint of the Object.
• It has the states.
• Single inheritance Vs Multiple inheritance, the former one is easy to understand, but the latter one is more power but complex.

Traits

• It is a group of similar functions
• It does not have a state.
• Multiple traits.
• Trait itself can have the implementation in the source code, it is just like copy & paste practice for code sharing. Of course, trait can have the abstract keyword in the methods to explicitly require the classes to implement those methods, like interface.

Interfaces

• It is a group of functions that the Objects MUST implement.
• It does not have a state.
• Multiple interfaces.
• Interface itself cannot have the implementation in the source code as compared with the trait.

Note:

• Abstract in PHP. 
• Basically, it defines the signature of the classes / methods. The abstract class cannot be initialized. It is being used to force the child classes to implement the methods.
• Interfaces in PHP.
• Abstract Class Vs Interface.
• Traits in PHP.