关于退选学程的代码，系统提示未定义变量

关于退选课程的代码，系统提示未定义变量。
退选课程提交后，系统提示错误，第15、35、38行有未定义的变量“xuehao” 请问我应该怎么修改？

<br /><br /><br /><meta http-equiv="Content-Type" content="text/html; charset=gb2312" /><br /><title>退选课题</title><br /><br /><br /><body><br /><?php <br />//######################退选课题界面3##########################<br /> include "config.php";<br /> include "header.php";?><br /><?php<br /> extract($_POST);<br />        $que<b>6本文来源gao@dai!ma.com搞$代^码!网7</b><pre>搞gaodaima代码

ry=”select * from $student_table where xuehao='”.$xuehao.”‘”;
mysql_query(“set names ‘GB2312′”);
$result=mysql_query($query);
$row=mysql_fetch_array($result);
$id=$row[‘id’];

$query1=”select number as sn,surplus as ssn from $jiaoshi_table where id=’$id'”;
mysql_query(“set names ‘GB2312′”);
$result1=mysql_query($query1);
$row1=mysql_fetch_array($result1);
if($row1[‘sn’]==1)
{
$query2=mysql_query(“update $jiaoshi_table set xuehao=’未选’ where id=’$id'”);
$query3=mysql_query(“update $jiaoshi_table set surplus=surplus+1 where id=’$id'”);
}
else
{
if(($row1[‘sn’]-$row1[‘ssn’])==1)
$query7=mysql_query(“update $jiaoshi_table set xuehao=’未选’ where id=’$id'”);
else
$query6=mysql_query(“update $jiaoshi_table set xuehao=replace(xuehao,concat(‘”.$xuehao.”‘,’\n’),”) where id=’$id'”);
$query8=mysql_query(“update $jiaoshi_table set surplus=surplus+1 where id=’$id'”);

$query5=”update $student_table set id=0 where xuehao='”.$xuehao.”‘”;
mysql_query(“set names ‘GB2312′”);
$result5=mysql_query($query5);
if($result5==true)
{
echo”

退选课题成功！“;
echo “<meta http-equiv=\”refresh\” content=\”2;url=back_keti.php\”>”;
exit;
}
else
{
echo”

退选出错，请返回重新退选

“;
echo “<meta http-equiv=\”refresh\” content=\”1;url=back_keti.php\”>”;
exit;
}
}

?>
<?php include “foot.php”;?>