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

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


namespace HH\Lib\Dict;

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

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

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