add xml data to http request

I am new in php programming and I would like to add data to my http request in xml format. I use curl to generate HTTP request using this PHP code :

<?php
      $fields = array(            'SERIAL_NUMBER ' => urlencode('123456'),
                            'HARDWARE_ID ' => urlencode('455FFG'),
                            'CODE ' => urlencode('99'),
                            //field goes here 
                );

      foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
      rtrim($fields_string, '&');

        $ch = curl_init("http://ift.tt/1w6tCvh");
        curl_setopt($ch, CURLOPT_HEADER, 1);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 0);                       
        $output = curl_exec($ch);
        curl_close($ch);
        echo $output;   

?>

I would like to add fields array [ SERIAL_NUMBER and HARDWARE_ID and CODE ] as data in http request in format xml Note : my server listen to port 10000 @ any help is appreciated