关于ajax 接收xml乱码有关问题

关于ajax 接收xml乱码问题？
一个PHP文件，从数据库中取出数据，用的xml返回给ajax，GB2312编码 get方式
PHP文件顶部已经定义
header("Content-Type: text/xml;charset=GBK");
但是在IE下还是空白的，其他浏览器正常，测试返回的数据换成英文的话是没问题的
有哪位遇到过这情况的，求指点……
一下为主要代码
ajax.php

<br />
header("Content-Type: text/xml;charset=GBK");<br />
$sheng=$_REQUEST['pro'];<br />
$info="";<br />
$query="select name from pre_common_district where upid=$sheng";<br />
$result=  mysql_query($query);<br />
$info="<res>";<br />
while($data=  mysql_fetch_assoc($result)){<br />
            $info.="<city>".$data['name']."</city>";<br />
        }<br />
$info.="</res>";<br />
echo $info;

ajax.js代码

<br>
if(http_request){<br>
                                 <br>
                                var url="myajax.php?pro="+$('sheng').selectedIndex;<br>
                                //var data="pro=";<br>
                                //window.alert(data);<br>
                                http_request.open("get",url,true);<br>
                                //http_request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");<br>
                                http_request.onreadystatechange=chuli;<br>
                                 <br>
                                http_request.send();<br>
                                 <br>
                        }<br>
 <br>
function chuli<br>
************<br>
var cities=http_request.responseXML.getElementsByTagName("city");<br>
 <br>
                                 <br>
                                 <br>
                                //把返回的城市动态添加到city控件 <div class="clear">
                 
              
              
        
            </div>