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 />        $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 $jiaoshi_table set xuehao<mark style="color:transparent">来4源gaodaimacom搞#代%码*网</mark><code>搞代gaodaima码</code>='未选' where id='$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 /><br />

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