Admin Cannot Update Student Profile - php

Question

currently im doing my final year project which is advising system for student. My problem is that admin cannot update a certain data of student. At my first try, it says that, admin done update student profile but when i check at the database it does not update at all. After that i change my code, it turns that the data is undefined variable and mysqli_fetch_array() expects parameter 1 to be mysqli_result, boolean given.
(AdminUpdateStd.php)
<?php
include("db.php");
$StdId = $_GET['id'];
$sql="SELECT * from member where id='$StdId'";
$result=mysqli_query($db,$sql);
while($rows=mysqli_fetch_array($result)){
$student_id=$rows[0];
$student_name=$rows[3];
$student_email=$rows[5];
$student_address=$rows[4];
$student_tel=$rows[6];
$intake=$rows[7];
$total_cred=$rows[8];
$advisor=$rows[9];
}
?>
<form action ="AdminUpdateProcess.php" method="POST">
<div id="reg-head" class="headrg" align="center"><strong>Profile Student <?php echo "$StdId"; ?></strong></div>
<table width="350" height="200" border="1" align="center" cellpadding="2" cellspacing="0" bgcolor="#FFF">
<input type="hidden" name="student_id" value=" <?php echo "$student_id"; ?>">
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name">Full Name:</div></td>
<td class="tl-4"><?php echo "$student_name"; ?></td>
</tr>
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name">Email:</div></td>
<td class="tl-4"><?php echo "$student_email"; ?></td>
</tr>
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name">Adress:</div></td>
<td class="tl-4"><?php echo "$student_address"; ?></td>
</tr>
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name">Tel Number:</div></td>
<td class="tl-4"><?php echo "$student_tel"; ?></td>
</tr>
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name">Intake:</div></td>
<td class="tl-4"><input type="text" name="intake" <?php echo "$intake"; ?>></td>
</tr>
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name">Total Credit Hour:</div></td>
<td class="tl-4"><input type="text" name="total_credit" <?php echo "$total_cred"; ?>></td>
</tr>
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name">Advisor:</div></td>
<td class="tl-4"><input type="text" name="advisor_name" <?php echo "$advisor"; ?>></td>
</tr>
<tr id="lg-1">
<td class="tl-1"><div align="left" id="tb-name"> </div></td>
<td class="tl-4"><input type="submit" name="Update" <?php echo "value='UPDATE'"; ?>></a>
<button type="button">View Profile</button></td>
</tr>
</table>
</form>
(AdminUpdateProcess.php)
<?php
include("db.php");
$intake=$_POST['intake'];
$total_cred=$_POST['total_credit'];
$advisor=$_POST['advisor_name'];
$StdId = $_POST['student_id'];
if (isset ($_POST['Update'])) {
$UpdateQuery ="UPDATE member SET intake = '$intake', total_credit = '$total_cred', advisor_name = '$advisor' WHERE student_id ='$StdId'";
$res = mysqli_query ($UpdateQuery) or die ("Could not update".mysqli_error());
}
?>