protected function toXML(DOMElement $parent) : DOMElement
    {
        $e = $parent->ownerDocument->createElementNS(Constants::NS_MD, $this->elementName);
        $parent->appendChild($e);

        if ($this->ID !== null) {
            $e->setAttribute('ID', $this->ID);
        }

        if ($this->validUntil !== null) {
            $e->setAttribute('validUntil', gmdate('Y-m-d\TH:i:s\Z', $this->validUntil));
        }

        if ($this->cacheDuration !== null) {
            $e->setAttribute('cacheDuration', $this->cacheDuration);
        }

        $e->setAttribute('protocolSupportEnumeration', implode(' ', $this->protocolSupportEnumeration));

        if ($this->errorURL !== null) {
            $e->setAttribute('errorURL', $this->errorURL);
        }

        Extensions::addList($e, $this->Extensions);

        foreach ($this->KeyDescriptor as $kd) {
            $kd->toXML($e);
        }

        if ($this->Organization !== null) {
            $this->Organization->toXML($e);
        }

        foreach ($this->ContactPerson as $cp) {
            $cp->toXML($e);
        }

        return $e;
    }