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

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

Just like filter, but your predicate can include the key as well as the value


namespace HH\Lib\Dict;

function filter_with_key<Tk as arraykey, Tv>(
  KeyedTraversable<Tk, Tv> $traversable,
  (function(Tk,Tv):bool) $predicate,
): dict<Tk, Tv>;

To use an async predicate, see Dict\filter_with_key_async().