function showHint(str)
{
    var xmlhttp;
    if (window.XMLHttpRequest)
     {// IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
     }
      else
     {// IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
     }
     xmlhttp.onreadystatechange=function()
    {
       if (xmlhttp.readyState==4 && xmlhttp.status==200)
      {
        document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
      }
  }
    xmlhttp.open("GET","keshi.php?q="+str,true);
    xmlhttp.send();
}

keshi.php:

<?php
/*防止恶意调用*/
define("TEST",'test');
// 引入文件
include_once 'mysql.func.php';
// 数据库初始化
connectMySQL();
selectDB();
setZiFuJi();

//获得来自 URL 的 q 参数
$q=$_GET["q"];
//如果 q 是数字或者数字字符串
if (is_numeric($q))
  {
    $q = intval($q);
    $hint="";
    $resultDKQ = queryDB("select name from table_dake where id=$q");
    $hint = '科室名字：<select name="ksname" >';
    while (!!$rowDKQ = fetchAssoc($resultDKQ))
    {
      $hint .= '<optgroup label='.$rowDKQ['name'].'>';
      $resultKSQ = queryDB("select table_dake.id,table_keshi.sid,table_keshi.name from table_dake,table_keshi where table_dake.name='{$rowDKQ['name']}' and table_keshi.sid=table_dake.id");
      while(!!$rowKSQ = fetchAssoc($resultKSQ))
      {
        $hint .= '<option style="padding-left:15px;" value ='. $rowKSQ['name'].'>'.$rowKSQ['name'].'</option>';
      }
      $hint .= '</optgroup>';
    }
  }
  // 不是数字
  else
  {
    $resultDK = queryDB("select table_dake.name from table_dake");
    $hint = '科室名字：<select name="ksname" >';
     while (!!$rowDK = fetchAssoc($resultDK))
    {
      $hint .= '<optgroup label='.$rowDK['name'].'>';
      $resultKS = queryDB("select table_dake.id,table_keshi.sid,table_keshi.name from table_dake,table_keshi where table_dake.name='{$rowDK['name']}' and table_keshi.sid=table_dake.id");
      while(!!$rowKS = fetchAssoc($resultKS))
      {
        $hint .= '<option style="padding-left:15px;" value ='. $rowKS['name'].'>'.$rowKS['name'].'</option>';
      }
      $hint .= '</optgroup>';
    }
  }
 $response=$hint;
//输出响应
echo $response;
?>