Warum erscheint immer ein undefinierter Index?

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<?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);

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	while( $arr=mysqli_fetch_array($result) )

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?>

	<form id="form1" method="post" action="update_ok_1.php">

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	<table border="1" width="400" align="center" cellpadding="0" cellspacing="0">

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		<tr>

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			<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">&nbsp;&nbsp;

				<input type="reset" name="sub2" value="清空">

			</td>

			</tr>

	<?php

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