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

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.