$xmlEncoder = new XmlEncoder();
$xml = '<?xml version="1.0"?><!DOCTYPE foobar SYSTEM "http://foo.bar"><foo>bar</foo>';
$xmlEncoder->decode($xml, 'xml', [XmlEncoder::DECODER_IGNORED_NODE_TYPES => [XML_DOCUMENT_TYPE_NODE]])

$rootNode = null;
$decoderIgnoredNodeTypes = $context[self::DECODER_IGNORED_NODE_TYPES] ?? $this->defaultContext[self::DECODER_IGNORED_NODE_TYPES];
foreach ($dom->childNodes as $child) {
    if (\in_array($child->nodeType, $decoderIgnoredNodeTypes, true)) {
        continue;
    }
    if (\XML_DOCUMENT_TYPE_NODE === $child->nodeType) {
        throw new NotEncodableValueException('Document types are not allowed.');
    }
    if (!$rootNode) {
        $rootNode = $child;
    }
}