public function get_form_fields() {
        if (empty($this->type)) {
            return '';
        }

        $result  = '<input type="hidden" name="gpr_type" value="'.$this->type.'" />';

        if (!empty($this->plugin)) {
            $result .= '<input type="hidden" name="gpr_plugin" value="'.$this->plugin.'" />';
        }

        if (!empty($this->courseid)) {
            $result .= '<input type="hidden" name="gpr_courseid" value="'.$this->courseid.'" />';
        }

        if (!empty($this->userid)) {
            $result .= '<input type="hidden" name="gpr_userid" value="'.$this->userid.'" />';
        }

        if (!empty($this->groupid)) {
            $result .= '<input type="hidden" name="gpr_groupid" value="'.$this->groupid.'" />';
        }

        if (!empty($this->page)) {
            $result .= '<input type="hidden" name="gpr_page" value="'.$this->page.'" />';
        }
        return $result;
    }