class Ajax{

  constructor(url, method, data, callback_suc, callback_err, callback_run){
   this.RT = true;//默认为异步请求
   this.url = url;
   this.method = method || "POST";
   this.data = data || "";
   this.callback_suc = callback_suc || function () {};
   this.callback_err = callback_err || function () {};
   this.callback_run = callback_run || function () {};
   if(!this.url){this.callback_err(); return;}
   this.createRequest();
  }

  createRequest(){
   let xhr = new XMLHttpRequest();
   xhr.onreadystatechange = (e)=>{this.run(e);}
   xhr.open(this.method, this.url, this.RT);
   xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
   xhr.send(this.data);
  }

  run(e){
   this.callback_run(e);
   if(e.target.readyState !== 4 || e.target.status !== 200){return;}
   this.callback_suc(e);
  }

 }
 //调用:
 new Ajax(
  "./main.php", //url：请求地址
  "POST", //method：请求方法
  "data=3&sb=2",//data：传递数据
  (e)=>{//callback_suc：请求完成 回调函数
   document.write(e.target.responseText);//3
  },
  (e)=>{},//callback_err：请求错误 回调函数
  (e)=>{}//callback_run：请求中 回调函数
 )

<"data为空";
 $sb = isset($_POST['sb']) "sb为空";
 //向客户端返回数据
 echo $data." ".$sb;
?>