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

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