var url = 'http://grupoweb.upf.edu/~wet/web/include/script.php';

url += '?url=' + encodeURIComponent(document.URL); 
url += '&ref=' + encodeURIComponent(document.referrer);
url += '&sync_num=' + sync_num;

document.write("<img src=\"" + url + "\" height=\"0\" width=\"0\">");