AJAX实现二级级联菜单，用PHP完成，客户端代码：
<!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;i<content.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;i<a.l
