public function retrieve_group_ids(string $id_option = null, string $id_group): ?array
  {
    return $this->_retrieve_ids($id_option, null, $id_group);
  }
public function retrieve_group_ids(string $id_option = null, string $id_group): ?array
  {
    return $this->_retrieve_ids($id_option, null, $id_group);
  }
public function retrieve_group_ids ( string $ id_option = null , string $ id_group ) : ? array { return $ this -> _retrieve_ids ( $ id_option , null , $ id_group ) ; }
Returns preferences' IDs from the option's ID and the given group ID

@param null|string $id_option
@param string $id_group
@return array|null
Returns preferences IDs from the option s ID and the given group ID