public function toXML(\DOMNode $parentElement = null) : DOMElement
    {
        if ($parentElement === null) {
            $document = DOMDocumentFactory::create();
            $parentElement = $document;
        } else {
            $document = $parentElement->ownerDocument;
        }

        $root = $document->createElementNS(Constants::NS_SAML, 'saml:'.'Assertion');
        $parentElement->appendChild($root);

        /* Ugly hack to add another namespace declaration to the root element. */
        $root->setAttributeNS(Constants::NS_SAMLP, 'samlp:tmp', 'tmp');
        $root->removeAttributeNS(Constants::NS_SAMLP, 'tmp');
        $root->setAttributeNS(Constants::NS_XSI, 'xsi:tmp', 'tmp');
        $root->removeAttributeNS(Constants::NS_XSI, 'tmp');
        $root->setAttributeNS(Constants::NS_XS, 'xs:tmp', 'tmp');
        $root->removeAttributeNS(Constants::NS_XS, 'tmp');

        $root->setAttribute('ID', $this->id);
        $root->setAttribute('Version', '2.0');
        $root->setAttribute('IssueInstant', gmdate('Y-m-d\TH:i:s\Z', $this->issueInstant));

        $issuer = $this->issuer->toXML($root);

        $this->addSubject($root);
        $this->addConditions($root);
        $this->addAuthnStatement($root);
        if ($this->getRequiredEncAttributes() === false) {
            $this->addAttributeStatement($root);
        } else {
            $this->addEncryptedAttributeStatement($root);
        }

        if ($this->signatureKey !== null) {
            Utils::insertSignature($this->signatureKey, $this->certificates, $root, $issuer->nextSibling);
        }

        return $root;
    }