xpath and curl

[dflow]

how would i query with xpath a specific node with city_id=2

//city_list
<cities>
<city>
<city_name>Barcelona</city_name>
<city_id>2</city_id>
</city>
<cities>


$ch = curl_init("http://example.com/citylist.php?rt=".$rt."");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_HEADER, 0);
$data = curl_exec($ch);
curl_close($ch);
$xmlcitylist = new SimpleXmlElement($data, LIBXML_NOCDATA);