function  creatXmlHttp(){var xmlrequest=null;try　　{xmlrequest= new XMLHttpRequest();}catch　　{try{xmlrequest=new ActiveXObject("Microsoft.XMLHTTP");}catch　　{alert("sorry.your explore don't support this function");}return xmlrequest;
}functon $get(id){return document.getElementById(id);   //.value取节点值
}

<?php
header("Content-Type：text/html;charset=utf-8");  //此处与返回格式对应
header("Cache-Control：no-cache");
$cityList=$_POST["cityList"];
for(i=0;i<count(cityList);i++){//暂时不操作数据库if(cityList(i)=="city1")　　{echo"{"subcity11":"city1","subcity12":"city2","subcity13":"city3"}";}　　else if(cityList(i)=="city2")　　{//json $res.='{"cityname":'.$cities[$i].',"price":'.rand(500,1500).'}';echo"{"subcity11":"city4","subcity12":"city5","subcity13":"city6"}";   }
}
?>

<select id="pro" οnchange="sendRequest();">
<option value=""></option>
<option value="city1">city1</option>
...
<select/>
<select id="city" value="">
...
<select/><script type="text/script" language="javascript">
function sendRequest()
{var xmlRequest=creatXmlHttp();//注意用法var data="city[]=city1&city[]=city2&city[]=city3";var url="xxxpro.php?cityList="+data;xmlRequest.setRequestHeader("Content-Type:");xmlRequest.open("Post",url,true);xmlRequst.onreadystatechange=profun;    //一个是函数的引用，一个是赋函数的地址。profun带不带（）的区别xmlRequest.send(data);
}
function profun()
{if(xmlRequest.readystate=4 && xmlRequest.status=200){$get("city").length=0;var subCityList=eval("("+xmlRequest.responseText+")");   //responseXML,返回类型为xml对象（xml dom）var subcitys =xmlRequest.responseXML.getElementsByTagName;var val =mes[0].childNodes[0].nodeValue;for(var i=0;i<subcitys;i++){var city_name=subCityList[i].childNodes[0].subcity11;var subcity=document.createElement("option");//注意语法subcity.value=city_name;subcity.innerText=city_name;$get("city").appendChild(subcity);...}}
}</script>