ajax-(myXmlHttpRequest.responseXML返回值为空问题

ajaxphpjshtml

代码是这样的：
showCities.php:

<code>             <meta http-equiv="content-type" content="text/html; charset=utf-8">
<script type="text/javascript">function getXmlHttpObject(){    var xmlHttpRequest;    if(window.ActiveXObject){        xmlHttpRequest=new ActiveXObject("Microsoft.XMLHTTP");    }else{        xmlHttpRequest=new XMLHttpRequest;    }    return xmlHttpRequest;}    var myXmlHttpRequest="";    function getCities(){        myXmlHttpRequest=getXmlHttpObject();        if(myXmlHttpRequest){            var url="/ajax/showCitiesPro.php";            var data="province="+$('sheng').value;            myXmlHttpRequest.open("post",url,true);            myXmlHttpRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded");            myXmlHttpRequest.onreadystatechange=chuli;            myXmlHttpRequest.send(data);        }    }    function chuli(){        if(myXmlHttpRequest.readyState==4){            if(myXmlHttpRequest.status==200){                window.alert(myXmlHttpRequest.responseXML);            }        }    }    function $(id){        return document.getElementById(id);    }</script>            <select id="sheng" onchange="getCities();"><!--onchange="getCities();"-->    <option value="">---省---</option>    <option value="zhejiang">浙江</option>    <option value="jiangsu">江苏</option>    </select>    <select id="city">    <option value="">--城市--</option>    </select>     <select id="county">    <option value="">--县城--</option>    </select>    </code>

服务器代码是这样的：
showCityPro.php:

<code> <?php header("Content-Type:text/xml;charset=utf-8");    header("Cache-Control: no-cache");    //接收用户选择的省    $province=$_POST['province'];    echo "接收到的数据是:";    ?>    window.alert(myXmlHttpRequest.responseXML);接收到的一直是null</code>

<code>这是为什么？新手，所以没啥悬赏。。求大神帮助。我快被这个错误折磨致死了。。。</code>