Domain-Driven Design in PHP

Immutability

This is one of the most important aspects to grasp. Object values shouldn't be able to be altered over their lifetime. Because of this immutability, Value Objects are easy to reason and test and are free of undesired/unexpected side effects. As such, Value Objects should be created through their constructors. In order to build one, you usually pass the required primitive types or other Value Objects through this constructor.

Value Objects are always in a valid state; that's why we create them in a single atomic step. Empty constructors with multiple setters and getters move the creation responsibility to the client, resulting in the Anemic Domain Model, which is considered an anti-pattern.

It's also good to point out that it's ...