下拉菜单-求php两个下拉列表联动解决办法.

php下拉菜单数据库

选择科室后，对应员工姓名根据选择的科室获取，怎么做到。
两个列表的代码：

```

echo "-请选择-  ";
@include "conn.php"; //链接数据库
$gzh_num_result=mysql_query("select gzh_num , gzh_name from gongzhong where gzh_renum=0 order by gzh_num ;");
while($gzh_num_row=mysql_fetch_array($gzh_num_result))
{

list($gzh_num,$gzh_name)=$gzh_num_row;

    echo "".$gzh_name."  ";        //  echosuboption($gzh_num,@$_GET['gzh_num']);};?>               echo "-请选择-  ";      ".$yg_name."  ";};?>