This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as Vec\slice() in Facebook's www repository.

Returns a new vec containing the subsequence of the given Traversable determined by the offset and length


namespace HH\Lib\Vec;

function slice<Tv>(
  Container<Tv> $container,
  int $offset,
  ?int $length = null,
): vec<Tv>;

If no length is given or it exceeds the upper bound of the Traversable, the vec will contain every element after the offset.

To take only the first $n elements, see Vec\take(). To drop the first $n elements, see Vec\drop().