Three-level linkage of provinces, cities and counties with mysql+ajax

In PHP development, I want to write a mysql+ajax three-level linkage between provinces, cities and counties. Everything has been written, and I am just waiting to return data in xml or json format.

The query is: $sql="select * from cities where sid=1";//1 is the ID of Jiangsu Province. Needless to say this

$res=mysql_query($sql);

How to deal with the following, use a while loop or how to do it? The xml format I want is similar to

NanjingSuzhou

Reprinted from other people’s questions and answers. . .

I will send you the code including the database once to express my gratitude! ! !
php page:
//The two sentences here are very important. The first sentence tells the browser that the data returned is in xml format
header("Content-Type: text/html;charset=utf-8") ;
//Tell the browser not to cache data
header("Cache-Control: no-cache");
$conn=mysql_connect("localhost","root","123");
mysql_select_db("ajax") ;
mysql_query("set names utf8");

if(isset($_POST['sid'])){
$sid=$_POST['sid'];
//file_put_contents("D:/a. txt",$sid);
$sql_shi="select shi from shi where sid=$sid";
$res_shi=mysql_query($sql_shi);
$arr="";
while($row=mysql_fetch_array($res_shi )){
 $arr[]=$row;
}
$a=json_encode($arr);
echo '{"aa":'.$a.'}';
}
?>
js page ：

html page:


 ---Province ---
 
 --city--< /option>
 
 --County--