浏览器中为我们提供了一个JS对象XMLHttpRequet，它可以帮助我们发送HTTP请求，并接受服务端的响应。
var xhr = new XMLHttpRequest(); //暂不考虑兼容
方法：
open("get/post", url, true/false);
//有参数则k=v&k1=v1这种形式
send(null);
属性：
//代表请求状态，不断变化，为4时，请求结束
readyState
//响应的内容
responseText
//响应的状态码200，403，404
status
//状态文字
statusText
事件：
//当readyState变化时会触发此事件
onreadystatechange = function() {};
<!DOCTYPE html>
<html>
<head>
<title>ajax</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div id="box">
<input type="text" name="con" value="" id="con" />
</div>
</body>
<script type="text/javascript">
var ipt = document.getElementById("con");
ipt.onblur = function () {
var con = this.value;
//创建XHR对象
var xhr = new XMLHttpRequest();
//设置请求URL
var url = "./ajax.php?con=" + con;
//设置XHR对象readyState变化时响应函数
xhr.onreadystatechange = function () {
//readyState是请求的状态，为4表示请求结束
if (xhr.readyState == 4) {
//responseText服务器响应的内容
alert("服务器响应数据：" + this.responseText);
}
};
//打开链接
xhr.open("get", url, true);
//发送请求
xhr.send(null);
}
</script>
</html>
<?php $con = !empty($_GET[‘con‘]) ? trim($_GET[‘con‘]) : ‘没有数据‘; echo $con;
<!DOCTYPE html>
<html>
<head>
<title>ajax</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div id="box">
<input type="text" name="name" value="" id="name" />
<input type="password" name="pwd" value="" id="pwd" />
<input type="submit" name="sub" value="提交" id="sub" />
</div>
</body>
<script type="text/javascript">
var sub = document.getElementById("sub");
sub.onclick = function () {
var name = document.getElementById("name").value;
var pwd = document.getElementById("pwd").value;
//创建XHR对象
var xhr = new XMLHttpRequest();
//设置请求URL
var url = "./ajax.php";
//设置XHR对象readyState变化时响应函数
xhr.onreadystatechange = function () {
//readyState是请求的状态，为4表示请求结束
if (xhr.readyState == 4) {
//responseText服务器响应的内容
alert("服务器响应数据：" + this.responseText);
}
};
//打开链接
xhr.open("post", url, true);
//设置请求头部
xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
//发送请求
xhr.send("name=" + name + "&pwd=" + pwd);
}
</script>
</html>
<?php $name = !empty($_POST[‘name‘]) ? trim($_POST[‘name‘]) : ‘没有数据‘; $pwd = !empty($_POST[‘pwd‘]) ? trim($_POST[‘pwd‘]) : ‘没有数据‘; echo ‘用户名：‘, $name, ‘密码：‘, $pwd;
<!DOCTYPE html>
<html>
<head>
<title>ajax</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div id="box">
<select id="city"></select>
<input type="button" value="获取" id="get" />
</div>
</body>
<script type="text/javascript">
var get = document.getElementById("get");
var city = document.getElementById("city");
get.onclick = function () {
//创建XHR对象
var xhr = new XMLHttpRequest();
//设置请求URL
var url = "./ajax.php";
//设置XHR对象readyState变化时响应函数
xhr.onreadystatechange = function () {
//readyState是请求的状态，为4表示请求结束
if (xhr.readyState == 4) {
//responseText服务器响应的内容
//通过eval把传来的json字符串转成对象
var data = eval(this.responseText);
var str = "";
for(var ix in data) {
str += "<option value=‘" + data[ix].id + "‘>" + data[ix].name + "</option>";
}
city.innerHTML = str;
}
};
//打开链接
xhr.open("get", url, true);
//发送请求
xhr.send(null);
}
</script>
</html>
<?php
$data = array(
array(‘id‘ => 1, ‘name‘ => ‘上海‘),
array(‘id‘ => 2, ‘name‘ => ‘北京‘),
array(‘id‘ => 3, ‘name‘ => ‘深圳‘),
);
echo json_encode($data);
<!DOCTYPE html>
<html>
<head>
<title>ajax</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div id="box">
<div id="news"></div>
<input type="button" value="获取" id="get" />
</div>
</body>
<script type="text/javascript">
var get = document.getElementById("get");
var news = document.getElementById("news");
get.onclick = function () {
//创建XHR对象
var xhr = new XMLHttpRequest();
//设置请求URL
var url = "./ajax.php";
//设置XHR对象readyState变化时响应函数
xhr.onreadystatechange = function () {
//readyState是请求的状态，为4表示请求结束
if (xhr.readyState == 4) {
//responseXML服务器响应的内容
var data = this.responseXML;
var str = "";
var title = data.getElementsByTagName("title");
str += "<p>" + title[0].childNodes[0].nodeValue + "</p>";
str += "<p>" + title[1].childNodes[0].nodeValue + "</p>";
str += "<p>" + title[2].childNodes[0].nodeValue + "</p>";
news.innerHTML = str;
}
};
//打开链接
xhr.open("get", url, true);
//发送请求
xhr.send(null);
}
</script>
</html>
<?php
header(‘Content-Type: text/xml;charset=utf-8‘);
$xml = <<<EOD
<?xml version="1.0" encoding="utf-8"?>
<news>
<title>111</title>
<title>222</title>
<title>333</title>
</news>
EOD;
echo $xml;
原文：http://www.cnblogs.com/jkko123/p/6352080.html