Why does undefined index always appear? How to solve it?
<?php
include("connect/connect.php"); $xh=$_GET['xh'];
$sql="select xh,xm,xb,zymc from xsjbxxb where xh=".$xh;/lect host,user from user
$result=mysqli_query($conn,$sql);
while( $arr=mysqli_fetch_array($result) )
{ ?>
<form id="form1" method="post" action="update_ok_1.php">
<table border="1" width="400" align="center" cellpadding="0" cellspacing="0">
<tr>
<td width="100">学号:</td>
<td><input type="text" name="xh" value="<?php echo $arr[0]; ?>"readonly></input></td>
</tr>
<tr>
<td>姓名:</td>
<td><?php echo $arr[1]; ?></td>
</tr>
<tr>
<td>性别:</td>
<td><?php echo $arr[2]; ?></td>
</tr>
<tr>
<td>专业名称:</td>
<td>
<?php echo $arr[3]; ?>
</td>
</tr>
<tr>
<td colspan="2" align="center"><input type="submit" name="sub1" value="添加">
<input type="reset" name="sub2" value="清空">
</td>
</tr>
<?php
}
?>
</form>
</table>
</body>
