This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as Dict\sort() in Facebook's www repository.

Returns a new dict sorted by the values of the given KeyedTraversable


namespace HH\Lib\Dict;

function sort<Tk as arraykey, Tv>(
  KeyedTraversable<Tk, Tv> $traversable,
  ?(function(Tv,Tv):int) $value_comparator = null,
): dict<Tk, Tv>;

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

To sort by some computable property of each value, see Dict\sort_by(). To sort by the keys of the KeyedTraversable, see Dict\sort_by_key().