php中用ajax实现二级级联（甚至多级级联）

html中：

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省份：
<select name="province_id" onChange="getfid01('showfid01')" style="border:1px solid #AEBBC4;width:150px;height:260px;" multiple>
    <option value="0">--请选择省份--</option>
    <?php
        include('../conn.php');
        $sql="select * from `table` order by `pid` desc";
        $query=mysql_query($sql,$conn);
        while($row=mysql_fetch_array($query)){
    ?>
        <option value="<?php echo $row['id']?>"><?php echo $row['title']?></option>
    <?php
        }
    ?>
</select>
城市：
<select name="city_id" id="showfid01" style="border:1px solid #AEBBC4;width:150px;height:260px;" multiple></select>


js代码：

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<script type="text/javascript">
<!--
var reobj = null;
var http_request = false;
if(window.XMLHttpRequest){
http_request=new XMLHttpRequest();
    if(http_request.overrideMimeType){
    http_request.overrideMimeType("text/xml");
    }
}
else if(window.ActiveXObject){
    try{
    http_request=new ActiveXObject("Msxml2.XMLHttp");
    }catch(e){
      try{
      http_request=new ActiveXobject("Microsoft.XMLHttp");
      }catch(e){}
      }
    }
   function send_request(url){
    if(!http_request){
    window.alert("创建XMLHttp对象失败！");
    return false;
    }
    http_request.open("GET",url,true);
    http_request.onreadystatechange=proces-s-request;
    http_request.send(null);
}
function proces-s-request(){
   if(http_request.readyState==4){
     if(http_request.status==200){
     document.getElementById(reobj).innerHTML=http_request.responseText;
}
else{
     alert("您所请求的页面不正常！");
     }
   }
}
                                 
function getfid01(obj){
    var obj=obj;
    var province_id=document.updoc2013.province_id.value;
    document.getElementById(obj).innerHTML="<option>loading...</option>";
    send_request("../ajax/get_city.php?province_id="+province_id);
    reobj=obj; 
}
//-->
</script>


后台php代码（注意编码必须是UTF-8，否则乱码）：

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<?php
        $province_id=$_GET['province_id'];
        if(!empty($province_id) && $province_id !='0'){
            include('../conn.php');
            $sql="select * from `city_table` where `fid`='$province_id' order by `pid` desc";
            $query=mysql_query($sql,$conn);
            while($row=mysql_fetch_array($query)){
?>
            <option value="<?php echo $row['id']?>"><?php echo $row['title']?></option>
<?php
        }
    }
?>