public static function getSchemaVersion($xml)
    {
        $doiXML = new \DOMDocument();
        $doiXML->loadXML($xml);

        $resources = $doiXML->getElementsByTagName('resource');
        $theSchema = 'unknown';
        if ($resources->length > 0) {
            if (isset($resources->item(0)->attributes->item(0)->name)) {
                $theSchema = substr($resources->item(0)->attributes->item(0)->nodeValue,
                    strpos($resources->item(0)->attributes->item(0)->nodeValue,
                        "/meta/kernel") + 5);
            }
        }

        return $theSchema;
    }