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

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

Returns a new dict containing only the keys found in both the input container and the given Traversable


namespace HH\Lib\Dict;

function select_keys<Tk as arraykey, Tv>(
  KeyedContainer<Tk, Tv> $container,
  Traversable<Tk> $keys,
): dict<Tk, Tv>;

The dict will have the same ordering as the $keys Traversable.