我是初学者，请教下大家，这是我的一个考试系统的前台页面代码：
<?php session_start();?>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>在线考试系统</title>
<style type="text/css">
<!--
body {
margin-left: 0px;
margin-top: 0px;
margin-right: 0px;
margin-bottom: 0px;
}
.style6 {color: #FFFFFF}
.STYLE7 {color: #FFFFFF; font-size: 12px; }
-->
</style></head>
<body>
/*所设的热点*/
<map name="Map">
<area shape="rect" coords="193,26,273,52" href="index.php?online=考生注册">
<area shape="rect" coords="311,25,387,50" href="index.php?online=考生登录">
<area shape="rect" coords="433,27,514,52" href="index.php?online=密码修改">
<area shape="rect" coords="553,28,629,49" href="index.php?online=成绩查询">
<area shape="rect" coords="667,27,746,50" href="index.php?online=开始考试">
<area shape="rect" coords="795,26,870,49" href="tc_dl.php">
</map>
<table width="1002" border="0" cellspacing="0" cellpadding="0">
<tr>
<td><table width="1002" border="0">
<tr>
<td><img src="images/bg_2.jpg" width="1002" height="142"></td>
</tr>
</table></td>
</tr>
</table>
<table width="1002" border="0" cellspacing="0" cellpadding="0">
<tr>
<td><img src="images/ban1.jpg" width="1002" height="72" border="0" usemap="#Map"></td>
</tr>
</table>
<table width="1002" height="143" border="0" cellpadding="0" cellspacing="0">
<tr>
<td width="111" valign="top" bgcolor="#F0EFEB"> </td>
<td width="778" valign="top" bgcolor="#F0EFEB"><table width="778" border="0" align="center" cellpadding="0" cellspacing="0">
<tr>
<td height="26" background="images/bg_4.jpg"><table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td width="30%"> </td>
<td width="70%" height="26"><span class="STYLE7">当前位置：在线考试系统 > <?php echo $online;?></span></td>
</tr>
</table></td>
</tr>
<tr>
<td height="350" align="center" valign="top" bgcolor="#FFFFFF"><?php
switch($online){
case "考生注册":
include("zhuce.php");
break;
case "考生登录":
include("denglu.php");
break;
case "密码修改":
include("xgmm.php");
break;
case "成绩查询":
include("cjcx.php");
break;
case "开始考试":
include("ksgz.php");
break;
case "选择考题":
include("jrkc.php");
break;
case "考试开始":
include("ksks.php");
break;
case "":
include("denglu.php");//总是只显示这个登陆模块
break;
}
?></td>
</tr>
<tr>
<td> </td>
</tr>
</table></td>
<td width="113" valign="top" bgcolor="#F0EFEB"> </td>
</tr>
</table>
</body>
</html>
点击各个热点区的按钮却不能把相应的php文件调入到该页下半部分，总是只显示登陆的那个模块，谁告诉我怎么回事？哪里出错了？？？
