This API is part of the Hack Standard Library, not HHVM itself.
Facebook Engineer?

This function is available as Dict\flip() in Facebook's www repository.

Returns a new dict keyed by the values of the given KeyedTraversable and vice-versa


namespace HH\Lib\Dict;

function flip<Tk, Tv as arraykey>(
  KeyedTraversable<Tk, Tv> $traversable,
): dict<Tv, Tk>;

In case of duplicate values, later keys overwrite the previous ones.