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

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

Returns a new dict containing only the keys for which the given predicate returns true


namespace HH\Lib\Dict;

function filter_keys<Tk as arraykey, Tv>(
  KeyedTraversable<Tk, Tv> $traversable,
  ?(function(Tk):bool) $key_predicate = null,
): dict<Tk, Tv>;

The default predicate is casting the key to boolean.