Returns a subset of the current MutableSet starting from a given key up to, but not including, the element at the provided length from the starting key

public function slice(
  int $start,
  int $len,
): MutableSet<Tv>;

$start is 0-based. $len is 1-based. So slice(0,2) would return the elements at key 0 and 1.

The returned MutableSet will always be a proper subset of the current MutableSet.