Piwik\Tracker\Request::getForcedUserId PHP Method

    public function getForcedUserId()
    {
        $userId = $this->getParam('uid');
        if (strlen($userId) > 0) {
            return $userId;
        }
        return false;
    }

 /**
  * @param Request $request
  * @param Visitor $visitor
  * @param Action|null $action
  *
  * @return mixed|false
  */
 public function onExistingVisit(Request $request, Visitor $visitor, $action)
 {
     $forcedUserId = $request->getForcedUserId();
     if ($forcedUserId) {
         return $forcedUserId;
     }
     return false;
 }