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

Returns a new vec containing only the elements of the first Traversable that do not appear in the second one, where an element's identity is determined by the scalar function

namespace HH\Lib\Vec;

function diff_by<Tv, Ts as arraykey>(
    Traversable<Tv> $first,
    Traversable<Tv> $second,
  (function(Tv):Ts) $scalar_func,
): vec<Tv>;

For vecs that contain arraykey elements, see Vec\diff().