// Provide the XMLHttpRequest class for IE 5-6:
if( typeof XMLHttpRequest == "undefined" ) XMLHttpRequest = function() {
  try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch(e) {}
  try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch(e) {}
  try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch(e) {}
  try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {}
  throw new Error("This browser does not support XMLHttpRequest.");
};

var req_num = 0;

function ajax(url, vars, callbackFunction) {
  var request =  new XMLHttpRequest();
  request.open("POST", url, true);
  request.setRequestHeader("Content-Type",
                           "application/x-www-form-urlencoded");
  request.onreadystatechange = function() {
    if (request.readyState == 4) {
        callbackFunction(request.responseText);
    }
  };
  req_num++;
  request.send(vars);
}