function Inint_AJAX() 
{  
  try { return new ActiveXObject("Msxml2.XMLHTTP");    } catch(e) {} //IE
  try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {} //IE
  try { return new XMLHttpRequest();                   } catch(e) {} //Native Javascript
  alert("XMLHttpRequest not supported");
  return null;
}

function test(параметры) 
{
  var type = document.getElementById("type"); 
  var url = 'modules/test.php?what=' + type.value;

  var req = Inint_AJAX();

  req.onreadystatechange = function ()
  {
    if (req.readyState == 4) 
    {
      if (req.status == 200) 
      {
        var text = req.responseText.toString(); //return value
        //Выполнение действия с результатом php ответа text
      }
    }
  };
   

  req.open('GET', url, true); //make connection
  req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded;charset=cp1251'); // set Header
  req.send(null); //send value
}

<?php

If ($_GET['type'])
}
  echo($_GET['type']); //Отправка AJAX'у ответ
}
?>