When using curl it is better to check for all possible errors and finally try log the response to be sure that you got XML.

$resp = curl_exec($curl);
if($errno=curl_errno($curl)){
    echo "Connection error #$errno: ".curl_error($curl);
    die();
}
$info=curl_getinfo($curl);
if($info['http_code']>=400){
    echo "HTTP error {$info['http_code']}";
    die();
}
echo $resp;
//If you got normal XML here you can continue with parsing

Please show your XML to find out what's wrong with it.