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

Returns a new dict sorted by some scalar property of each value of the given KeyedTraversable, which is computed by the given function

namespace HH\Lib\Dict;

function sort_by<Tk as arraykey, Tv, Ts>(
    KeyedTraversable<Tk, Tv> $traversable,
  (function(Tv):Ts) $scalar_func,
  ?(function(Ts,Ts):int) $scalar_comparator = NULL,
): dict<Tk, Tv>;

If the optional comparator function isn't provided, the values will be sorted in ascending order of scalar key.