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？
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><br /><html xmlns="http://www.w3.org/1999/xhtml"><br /><head><br /><meta http-equiv="Content-Type" content="text/html; charset=gb2312" /><br /><title>退选课题</title><br /></head><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 /> $query="select * from $student_table where xuehao='".$xuehao."'";<br /> mysql_query("set names 'GB2312'");<br /> $result=mysql_query($query);<br /> $row=mysql_fetch_array($result);<br /> $id=$row['id'];<br /><br /> $query1="select number as sn,surplus as ssn from $jiaoshi_table where id='$id'";<br /> mysql_query("set names 'GB2312'");<br /> $result1=mysql_query($query1);<br /> $row1=mysql_fetch_array($result1);<br /> if($row1['sn']==1)<br /> {<br /> $query2=mysql_query("update $jiaoshi_table set xuehao='未选' where id='$id'");<br /> $query3=mysql_query("update $jiaoshi_table set surplus=surplus+1 where id='$id'");<br /> }<br /> else<br /> {<br /> if(($row1['sn']-$row1['ssn'])==1)<br /> $query7=mysql_query("update $jiaoshi_table set xuehao='未选' where id='$id'");<br /> else<br /> $query6=mysql_query("update $jiaoshi_table set xuehao=replace(xuehao,concat('".$xuehao."','\n'),'') where id='$id'");<br /> $query8=mysql_query("update $jiaoshi_table set surplus=surplus+1 where id='$id'");<br /> <br /> $query5="update $student_table set id=0 where xuehao='".$xuehao."'";<br /> mysql_query("set names 'GB2312'");<br /> $result5=mysql_query($query5);<br /> if($result5==true)<br /> {<br /> echo"<p align=\"center\"><font color=\"#FF0000\"><b><big>退选课题成功！</big></b></font>";<br /> echo "<meta http-equiv=\"refresh\" content=\"2;url=back_keti.php\">";<br /> exit;<br /> }<br /> else<br /> {<br /> echo"<p align=\"center\"><font color=\"#FF0000\"><b><big>退选出错，请返回重新退选</big></b></font></p>";<br /> echo "<meta http-equiv=\"refresh\" content=\"1;url=back_keti.php\">"; <br /> exit;<br /> }<br /> }<br /><br />?><br /><?php include "foot.php";?><br /></body><br /></html>
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