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

This function is available as Dict\map_with_key() in Facebook's www repository.

Returns a new dict where each value is the result of calling the given function on the original value and key


namespace HH\Lib\Dict;

function map_with_key<Tk as arraykey, Tv1, Tv2>(
  KeyedTraversable<Tk, Tv1> $traversable,
  (function(Tk,Tv1):Tv2) $value_func,
): dict<Tk, Tv2>;