js ajax

var ajax = new XMLHttpRequest();
ajax.open("get",'apply.php');
ajax.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
ajax.send(null);
ajax.onreadystatechange = function() {
  if ((ajax.readyState == 4) && (ajax.status == 200)) {
      //返回值
     var msg= ajax.responseText;
  }}