A PHP internals mailing list discussion about a proposed RFC to add literal scalar types to PHP, allowing function signatures to express exact allowed values like `0|1|2` or `'success'|'error'`. The RFC author argues literal types fill gaps that 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 handle most cases, backed enums cover scalar interop, and a non-exhaustive enum marker RFC could address open sets. The author concedes enums are better for named reusable sets but maintains literal types are uniquely suited for typed scalar data on the wire, especially in future array shape typing.
