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

Returns a vec containing the original dict split into chunks of the given size

namespace HH\Lib\Dict;

function chunk<Tk as arraykey, Tv>(
    KeyedTraversable<Tk, Tv> $traversable,
  int $size,
): vec<dict<Tk, Tv>>;

If the original dict doesn't divide evenly, the final chunk will be smaller.