Piwik\Common::mb_substr PHP Method

    public static function mb_substr($string, $start)
    {
        $length = func_num_args() > 2 ? func_get_arg(2) : self::mb_strlen($string);
        if (function_exists('mb_substr')) {
            return mb_substr($string, $start, $length, 'UTF-8');
        }
        return substr($string, $start, $length);
    }

 /**
  * @param Request $request
  * @param Visitor $visitor
  * @param Action|null $action
  * @return mixed
  */
 public function onNewVisit(Request $request, Visitor $visitor, $action)
 {
     $information = $this->getReferrerInformationFromRequest($request);
     if (!empty($information['referer_keyword'])) {
         return Common::mb_substr($information['referer_keyword'], 0, 255);
     }
     return $information['referer_keyword'];
 }