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

This function is available as Keyset\diff() in Facebook's www repository.

Returns a new keyset containing only the elements of the first Traversable that do not appear in any of the other ones


namespace HH\Lib\Keyset;

function diff<Tv1 as arraykey, Tv2 as arraykey>(
  Traversable<Tv1> $first,
  Traversable<Tv2> $second,
  Traversable<Tv2> ...$rest,
): keyset<Tv1>;