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

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.