per il post basta cambiare la funzione in questo modo:

codice:
function xxx() { 
  var HTTPobj = new RSobj();
  var postData = "?var1=value1&var2=value2& ... &varn = valuen";

  HTTPobj.open("POST", "x.php", true);
  HTTPobj.setRequestHeader("Method", "POST x.php HTTP/1.1");
  HTTPobj.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
  HTTPobj.setRequestHeader("Content-length", postData.length);
  HTTPobj.setRequestHeader("Connection", "close");  

  HTTPobj.onreadystatechange = function() { cut }
  //modifiche eventuali al documento
  HTTPobj.send(postData);
}