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

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

Merges multiple KeyedTraversables into a new dict


namespace HH\Lib\Dict;

function merge<Tk as arraykey, Tv>(
  KeyedTraversable<Tk, Tv> $first,
  KeyedTraversable<Tk, Tv> ...$rest,
): dict<Tk, Tv>;

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