What are PHP 8.1 enums?

Enums (PHP 8.1) are a first-class way to define a type that has a fixed set of possible values. A pure enum has only case names: enum Status { case Active; case Inactive; case Pending; }. A backed enum associates each case with a scalar value: enum Color: string { case Red = "red"; case Blue = "blue"; }. Access cases as singletons: Status::Active. Backed enums can be created from a value: Color::from("red") (throws on invalid) or Color::tryFrom("red") (returns null). Enums can implement interfaces, have methods, and use constants. They replace the common pattern of class constants for representing finite sets of values.