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

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

Returns a new dict containing only the entries of the first KeyedTraversable whose keys do not appear in any of the other ones


namespace HH\Lib\Dict;

function diff_by_key<Tk1 as arraykey, Tk2 as arraykey, Tv>(
  KeyedTraversable<Tk1, Tv> $first,
  KeyedTraversable<Tk2, mixed> $second,
  KeyedTraversable<Tk2, mixed> ...$rest,
): dict<Tk1, Tv>;