Readonly properties and readonly classes were introduced in PHP versions 8.1 and 8.2 respectively...but what are they?

A readonly property is one that can be initialized once and cannot be modified afterwards...therefore they are immutable properties. You can make individual properties readonly with this syntax

readonly public string $someProperty;

If, however, you want to make all of the properties on the class readonly, you can simply make the class readonly, like so:

readonly class SomeClass {}

The properties on our UserRegistration DTO should not change once initialized so let's make the class readonly

Branch: https://github.com/GaryClarke/learn-oophp/tree/118-readonly-properties-and-classes