Difference between Trait and an Abstract Class in PHP

Question

I recently came across Traits in PHP and I'm trying to understand them. During my research I stumbled upon this Stack Overflow question: Traits vs. Interfaces. The accepted answer mentions the following:

An interface defines a set of methods that the implementing class must implement.

When a trait is use'd the implementations of the methods come along too--which doesn't happen in an Interface.

So far so good but this sounds exactly like the difference between an interface and an abstract class to me. So this raises a follow-up question for me:

• What is the difference between a Trait and an Abstract Class in PHP?

I am aware that I can extend from only one abstract class and on the other hand use any amount of traits. But is this really the only difference? I still don't completely understand traits and its use.

Answer 1

Traits allow you to share code between your classes without forcing you into a specific class hierarchy. Say you want all your classes to have the convenient utility method foo($bar); without traits you have two choices:

• implement it individually with code redundancy in each class
• inherit from a common (abstract) ancestor class

Both solution aren't ideal, each with their different tradeoffs. Code redundancy is obviously undesirable, and inheriting from a common ancestor makes your class hierarchy design inflexible.

Traits solve this problem by letting you implement foo($bar) in a trait which each class can "import" individually, while still allowing you to design your class hierarchy according to business logic requirements, not language necessities.

Answer 2

Not exactly... Let's quote official documentation for this purpose:

A Trait is similar to a class, but only intended to group functionality in a fine-grained and consistent way. It is not possible to instantiate a Trait on its own. It is an addition to traditional inheritance and enables horizontal composition of behavior; that is, the application of class members without requiring inheritance.

So Traits are used for composition purposes to enable the class to perform some logic/behavior. If you're inheriting from another/abstract class, it's usually for purposes of polymorphism and you get a distinct inheritance/class hierarchy, which may or may not be desirable.

I think it all depends on the context, on the architecture and on what exactly are you trying to do.