Enums and Pattern Matching

An order is pending, shipped, or cancelled. Never anything else. A playing card belongs to one of four suits, full stop. A traffic light is red, amber, or green. Programs are full of values like these, and for a long time PHP had no proper way to say so.

Before PHP 8.1 you reached for a string, 'shipped', or an integer constant, and hoped. Nothing stopped a colleague from typing 'shiped' somewhere. Nothing told you, in any one place, what the full list of valid values even was. The rule lived in your head, and heads forget.

An enum turns that list into a real type, checked by the engine, that can only ever hold one of the cases you declared. It is the difference between a text field where anyone can type anything and a knob with three positions engraved in the metal.

You met match briefly in Control Flow, sizing up an HTTP status code, and classes have given shape to your data since Chapter 5. Enums sit right between the two: they look like a small class, and match was made to read them. The chapter ends on the nullsafe operator, ?->, a cousin from the same family. It deals with a set of exactly two possibilities, something or nothing, without a defensive if in front of every access.