使用原生JS实现简单的ajax

         //浏览器端    
         var xhr = new XMLHttpRequest;
         xhr.open('get', './bb.php?id=4');
         xhr.send();
         xhr.onreadystatechange = function() {
                  if(xhr.readyState == 4 && xhr.status == 200)  //响应完成并且响应码为200
                  alert(xhr.responseText);
         }

　　　    //服务器端
         //example.php
         <?php
         echo '欢迎' . $_GET['id'];

//前端
var xhr = new XMLHttpRequest;
xhr.open('post', './example.php');
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.send('id=4&num=5');
xhr.onreadystatechange = function() {
         if(xhr.readyState == 4 && (xhr.status == 200 || xhr.status ==304))  //响应完成并且响应码为200或304
         alert(xhr.responseText);
}

//后端
//example.php
<?php
echo '欢迎'. $_POST['id'];
echo '欢迎'. $_POST['num'];