Piwik\Plugins\Referrers\API::getCleanKeyword PHP Method

    public static function getCleanKeyword($label)
    {
        return $label == self::LABEL_KEYWORD_NOT_DEFINED ? self::getKeywordNotDefinedString() : $label;
    }

 public function getKeyword()
 {
     $keyword = $this->details['referer_keyword'];
     if ($this->getReferrerType() == 'search') {
         $keyword = API::getCleanKeyword($keyword);
     }
     return urldecode($keyword);
 }