A PHP internals mailing list discussion around a proposed RFC to add literal scalar types to PHP, allowing function signatures to express exact values like `0|1|2` or `'success'|'error'` as types. The RFC author argues literal types fill gaps enums cannot: describing existing scalar APIs without breaking callers, handling open value sets that grow without BC breaks, and enabling direct scalar interop (JSON round-trips, array keys, === comparisons with raw values). Critics counter that enums already cover most use cases, backed enums handle scalar interop, and a non-exhaustive enum marker RFC could address open sets. The author concedes enums are better for named, reusable, behaviour-carrying sets, but maintains literal types are uniquely suited for typed scalar data on the wire, especially in future array shape typing scenarios.
