How to use an XML string to find its certain element values? i used this code:

    10: $php_var = json_decode($json, 1);
    11  $serializer=new XML_Serializer();
    12: if($serializer->serialize($php_var))
    13: xml_string=htmlspecialchars($serializer->getSerializedData()); 
    14: echo $xml_string;
    15: $xml_obj = new DOMDocument();
    16: $xml_obj->loadXML($xml_string);
    17: $urls=$xml_obj->getElementsByTagName('url');

ama çalışmıyor ve ben var bir Uyarı: DOMDocument::loadXML() [domdocument.loadxml]: Start tag expected, '<' not found in Entity, line: 1 in /var/www/seo-tool/get_details.php on line 15