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

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>;