php高手！！急求回答，高分先送！！运行不出结果，，是不是那里有问题了，witch($online){⼀⼀ 问题就在这里

<table width="1002" border="0" cellspacing="0" cellpadding="0"> <tr> <td><img src="images/bg_1.jpg" width="1002" height="72" border="0" usemap="#Map"> <map name="Map"> <area shape="rect" coords="355,30,418,52" href="index.php? online="用户注册""> <area shape="rect" coords="443,30,508,50" href="index.php? online="用户登录""> <area shape="rect" coords="529,31,599,52" href="index.php? online=修改密码"> <area shape="rect" coords="618,29,680,53" href="index.php? online=成绩查询"> <area shape="rect" coords="706,29,773,53" href="index.php? online=进入考场"> <area shape="rect" coords="794,29,852,52" href="tc_dl.php"> </map></td> </tr></table><?phpswitch($online){// 问题就在这里：运行时，$online没有值，怎么获取$online的值， case "用户注册": include("zhuce.php"); break; case "用户登录": include("denglu.php"); break; case "修改密码": include("xgmm.php"); break; case "成绩查询": include("cjcx.php"); break; case "进入考场": include("ksgz.php"); break; case "选择考题": include("jrkc.php"); break; case "开始考试": include("ksks.php"); break; case "": include("jrkc.php"); break;}?>怎么获取 $online的值？？