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

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.