In a post to his site Mark Ragazzo looks at immutable objects - what they are and how they can be used in a PHP application with some "final" functionality.

In this short article we will see what immutable objects are and why we should consider to use them. Immutable object is an object that does not change its state after it was created. Immutable objects usually are very simple. You may already seen them as enum types or primitives like DateTimeImmutable.

Further in this article you will learn that making simple objects immutable may save significant amount of your time by making impossible to make certain types of mistakes.

He starts with a list of a few things to remember when implementing immutable objects (like using the "final" keyword) and problems that can come without them. He then gets into some examples, showing how to create immutable Address and Money objects and how to use them when you need to update/get values from the object. He also covers some common "accidental mutability" cases like leaking internal object references and inheritance problems.