...emitting as var for XP reflection. Only useful for consistency purposes by itself, because you can just as well omit the typehint alltogether from a technical point of view, but important for arrays:

public function headers(): array<string, mixed> {
  // The equivalent of putting `@return [:var]` into the apidocs
}

See https://wiki.php.net/rfc/mixed-typehint and https://docs.hhvm.com/hack/types/type-system#mixed