'Serialization of 'SimpleXMLElement' is not allowed when saving in Wordpress post_meta

I am working on an amazon affiliate wordpress page. For that I am using the aws_signed_request function to get the price and link from amazon.

Here is the aws_signed_request function returning the xml:

    function  aws_signed_request($region, $params, $public_key, $private_key, $associate_tag) {
    $method = "GET";
    $host = "ecs.amazonaws.".$region;
    $uri = "/onca/xml";

    $params["Service"]          = "AWSECommerceService";
    $params["AWSAccessKeyId"]   = $public_key;
    $params["AssociateTag"]     = $associate_tag;
    $params["Timestamp"]        = gmdate("Y-m-d\TH:i:s\Z");
    $params["Version"]          = "2009-03-31";

    ksort($params);

    $canonicalized_query = array();

    foreach ($params as $param=>$value)
    {
        $param = str_replace("%7E", "~", rawurlencode($param));
        $value = str_replace("%7E", "~", rawurlencode($value));
        $canonicalized_query[] = $param."=".$value;
    }

    $canonicalized_query = implode("&", $canonicalized_query);

    $string_to_sign = $method."\n".$host."\n".$uri."\n".
                            $canonicalized_query;

    /* calculate the signature using HMAC, SHA256 and base64-encoding */
    $signature = base64_encode(hash_hmac("sha256", 
                                  $string_to_sign, $private_key, True));

    /* encode the signature for the request */
    $signature = str_replace("%7E", "~", rawurlencode($signature));

    /* create request */
    $request = "http://".$host.$uri."?".$canonicalized_query."&Signature=".$signature;

    /* I prefer using CURL */
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,$request);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_TIMEOUT, 15);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);

    $xml_response = curl_exec($ch);

   if ($xml_response === False)
    {
        return False;
    }
    else
    {
        $parsed_xml = @simplexml_load_string($xml_response);