添加产品webservice prestashop时出错500

Good day, I have a problem

I use webservice to add products in prestashop, I have solved all the other issues by now, but this is getting me sick.

first of all, I use a cdata node for almost everything there, it goes good, but when it comes the description I have this code :

$xml = $webService -> get(array('url' => PS_SHOP_PATH . '/api/products?schema=blank'));

$resources = $xml -> children() -> children();$no = $node -> ownerDocument;
....
....
    $node -> appendChild($no -> createCDATASection($n_desc));
    $resources -> description -> language[0][0] = $n_desc;
    $resources -> description -> language[0][0]['id'] = $n_l_id;
    $resources -> description -> language[0][0]['xlink:href'] = PS_SHOP_PATH . '/api/languages/' . $n_l_id;
....
....
try {
        //print_r($xml);
        $opt = array('resource' => 'products');
        $opt['postXml'] = $xml -> asXML();
        $xml = $webService -> add($opt);
        return true;
    } catch (PrestaShopWebserviceException $ex) {
        //echo '<b>Error : '.$ex->getMessage().'</b>';
        //$trace = $ex->getTrace();
        //print_r($trace);
        return false;
    }

the description comes from:

$n_desc = $row['descrizione'];

and this is the response:

HTTP/1.1 500 Internal Server Error
Date: Wed, 26 Feb 2014 15:37:59 GMT
Server: Apache/2.2.16 (Debian)
Vary: Host,Accept-Encoding
X-Powered-By: PrestaShop Webservice
Access-Time: 1393429079
PSWS-Version: 1.5.6.1
Execution-Time: 0.026

then it comes all the xml sent, the description part is:

<description><language id="1" xlink:href="server.com/api/languages/1">&lt;p&gt;&lt;span style="font-size: 16px,"&gt;lorem ipsum dolor sit ...

I have try to encode it, real_escape, utf_encode, url encode... nothing.

edit of course if I send a description string like "description", all goes good.

does anyone has an idea? thanks in advance.