code: public function modifyRelationship($action, $user) {
          if (true === in_array($action, $this->_actions) && isset($user)) {
              return $this->_makeCall('users/' . $user . '/relationship', array('action' => $action), 'POST');
          }
          throw new InvalidParameterException('Error: modifyRelationship() - This method requires an action command and the target user id.');
      }
docs: Modify the relationship between the current user and the target user
  
  @param string $action                Action command (follow/unfollow/block/unblock/approve/deny)
  @param integer $user                 Target user ID
  @return mixed