This isn't exactly a sum type. Sum type is basically a Union, a proper algebraic data type, and PHP doesn't have algebraic data types (although, even before typ

This isn't exactly a sum type. Sum type is basically a Union, a proper algebraic data type, and PHP doesn't have algebraic data types (although, even before type hints it was a relatively common practice to "implement" them via PhpDoc... horrible, I know). Enum is basically a bunch of constants, for all practical purposes. There were enums in Java for a long time, long before languages with more-or-less proper type systems became mainstream, and everybody was fine with a rigid mess of type system Java has. So, to have what effectively is enum in PHP, I do just class Status { const STATUS_A = 'STATUS_A'; const STATUS_B = 'STATUS_B'; // and so on { So it looks like a new enum would be basically just a syntactic sugar for that. (And finally a way to make a type hint for $a = Status::STATUS_A.)