PHP Firebase input is inputting blank instead of the submitted value? - php

Question

I am working on a grading system but whenever I input a grade, it saves a blank instead of the submitted value.
The form is on the table itself and not on a separate page. I know doing it on a separate page will solve my problem but I want to do it on the table itself.
Like this
Here is the code for the table
<tr>
<th>Student Number</th>
<th>Course</th>
<th>Email</th>
<th>Name</th>
<th>Grade</th>
<th>Remark</th>
<th>Add/Edit</th>
<?php
$table = "Users";
$fetch = $database->getReference($table)->getValue();
if($fetch > 0)
{
foreach($fetch as $key => $row)
{
?>
<tr>
<td> <?= $row['Student_number']; ?> </td>
<td> <?= $row['Course']; ?> </td>
<td> <?= $row['Email']; ?> </td>
<td> <?= $row['Name']; ?> </td>
<?php
if(!$row['grade']
{
?>
<td><input type = "text" name = "grade" id = "grade"></input></td>
<td><button type = "submit" name = "update" value = "<?=$key?>"</button></td>
<td>N/A</td>
}
else
{
?>
<td><?=$row[grade];?></td>
<?php
}
}
}
?>
And here is the code for the input
if(isset($_POST[update]))
{
$key = $_POST[update];
$grade = $_POST[grade];
$postGrade = ['grade' => $grade,];
$table = 'Users;
$updateResult = $database->getReference($table)->update($postGrade);
}
I need help finding what's wrong with my code that makes the input a blank instead of the intended submitted value.