Jeg er ikke med på hvad du mener, men her er siden hvor opdateringerne sker.
(Koden er ikke særlig køn, den må også gerne kommenteres)
<?php
include'../includes/connection.php';
include'../includes/topp.php';
// You should call this first
session_start();
// Initialize the session value
$_SESSION['ELEV_ID'] = $_POST["ELEV_ID"];
$_SESSION['P_NAME'] = $_POST["P_NAME"];
echo $_POST["KLASSE"];
echo $_POST["P_NAME"];
?>
<form role="form" method="post" action="update_prover1.php">
<div class="card shadow mb-4">
<div class="card-header py-3">
<h4 class="m-2 font-weight-bold text-primary"><?PHP echo $_POST["KLASSE"];?></h4>
</div>
<div class="card-body">
<div class="table-responsive">
<table class="table table-bordered" id="dataTable" width="100%" cellspacing="0">
<thead>
<tr>
<th>Klasse</th>
<th>Navn</th>
<th>Efternavn</th>
<th>Prøve 1</th>
<th>Prøve 2</th>
<th>Gennemsnit</th>
<th>Klasse</th>
</tr>
<tr>
<th> <button type="submit" formmethod="post">Submit using POST</button></th>
</tr>
</thead>
<tbody>
<?php
$query = '
SELECT *
FROM `elevnavn` as `elevnavn1`
INNER JOIN `product` as `product1`
ON `elevnavn1`.`P_NAME` = `product1`.`P_NAME`
WHERE `elevnavn1`.`P_NAME`="'.$_POST["P_NAME"].'" AND KLASSE = "'.$_POST["KLASSE"].'"
';
$result = mysqli_query($db, $query) or die (mysqli_error($db));
while ($row = mysqli_fetch_assoc($result)) {
echo '<tr>';
echo '<td><input class="form-control" placeholder="First Name" name="ELEV_ID" value='.$row['ELEV_ID'].'></td>';
echo '<td><input class="form-control" placeholder="First Name" name="F_NAME" value='.$row['F_NAME'].'></td>';
echo '<td><input class="form-control" placeholder="First Name" name="L_NAME" value='.$row['L_NAME'].'></td>';
echo '<td>'. $row['P1'].'</td>';
echo '<td>'. $row['P2'].'</td>';
echo '<td>'. $row['KLASSE'].'</td>';
echo '<td>'. $row['KLASSE'].'</td>';
echo '</tr> ';
}
?>
</tbody>
</table>
</div>
