var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() 
{
   if (this.readyState == 4 && this.status == 200)
      {

         res = this.responseText;
         document.write('response is '+res);

      }
};

data = "variable1=variable1value&variable2=variable2value";

xmlhttp.open("POST", "script.php", true);
xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xmlhttp.send(data);