AJAX 二级级联菜单实现代码

!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
html xmlns="http://www.w3.org/1999/xhtml">
head>
meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
title>无标题文档/title>
/head>
script type="text/javascript">
var xmlHttp;
var a =new Array();
function createXMLHttpRequest(){
if(window.ActiveXObject){
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
else if(window.XMLHttpRequest){
xmlHttp=new XMLHttpRequest();
}
}
function send_request(){
createXMLHttpRequest();
var year = document.getElementById("year");
var url = "check_2.php?page="+escape(year.value);
xmlHttp.open("GET",url,true);
xmlHttp.onreadystatechange = checkit;
xmlHttp.send(null);
}
function checkit(){
if(xmlHttp.readystate == 4){
if(xmlHttp.status == 200){
showChild();
}
}
}
function showChild(){
var xmlDoc=xmlHttp.responseXML;
var content=xmlDoc.getElementsByTagName("city");
for(var i=0;icontent.length;i++){
var y=content[i];
a[i]=y.childNodes[0].data;
}
show1();
}
function show1(){
var obj=document.getElementById("name");
var number=obj.length;
for(var j=obj.length-1;j>=0;j--){
obj.removeChild(obj.childNodes.item(j));
}
for(var i=0;ia.length;i++){
var opt=document.createElement("OPTION");
opt.text=a[i];
obj.add(opt);
}
}
/script>
body>
年份：select id="year" onchange="send_request()">
option value="0">请选择/option>
option value="1">1996-2006/option>
option value="2">1986-1995/option>
option value="3">1971-1985/option>
option value="4">1970以前/option>
/select>
子目录:select id="name">
option value="0">请选择/option>
/select>
/body>
/html>

?php
header('Content-type: text/xml');
$xml="?xml version='1.0' encoding='GB2312'?>";
$year = $_GET["page"];
$content = $xml."contents>";
if($year == "1"){
$content = $content."city>1/city>city>11/city>/contents>";
}
else if($year == '2'){
$content = $content."city>2/city>city>12/city>/contents>";
}
else if($year == '3'){
$content = $content."city>3/city>city>13/city>/contents>";
}
else if($year == '4'){
$content = $content."city>4/city>city>14/city>/contents>";
}
echo $content;
?>