Edit row in php - php

Question

I'm trying to build a forum. I have forum.php with a table that includes a row for the title, a row for the edit link and a row for the delete link. When I click the edit link, I am taken to edit.php where I have another form to insert the new topic title. When I click on "save new topic" button the row should be updated, however that's my problem, the title remains the same. I've been searching around this website (and the net in general) to find a solution to this but none seem to be working for me.
In forum.php I have this code:
<?php
while ($row = mysqli_fetch_assoc($result))
{
$subject = $row['subject'];
$id = $row['id'];
?>
<tr>
<th> <?php echo $row ['subject']?> </th>
<th>Edit</th>
<th>Delete</th>
</tr>
<?php
}
?>
In edit.php I have this code:
<div id="form">
<form id='edit' action='edit.php' method='post' >
<fieldset>
<legend>Edit Topic</legend>
<br />
<label for='name' >New Subject</label><br/>
<input type="text" id="subject" name="newsubject" /><br />
<br/>
<input type='hidden' name='id' value='<?php echo $id ?>'/>
<input type="submit" name="save" value="Save New Topic" />
</fieldset>
</form>
</div>
<?php
if (isset($_POST['save']))
{
$subject_save = $_POST['newsubject'];
require_once("db_connection.php");
$conn = connectToMySQL();
$id =$_POST['id'];
$query = "UPDATE tbl_topic SET subject = '$subject_save' WHERE id = 'id'";
$result = mysqli_query($conn, $query)
or die("Error in query: ".mysqli_error($conn));
header("Location: forum.php");
die();
}
?>

Answer 1

Write the query as
$query = "UPDATE tbl_topic SET subject = '$subject_save' WHERE id = $id";

Answer 2

if this not a mistake in typing
change $query = "UPDATE tbl_topic SET subject = '$subject_save' WHERE id = 'id'";
to $query = "UPDATE tbl_topic SET subject = '$subject_save' WHERE id = $id";

Answer 3

<div id="form">
<form id='edit' action='edit.php' method='post' >
<fieldset>
<legend>Edit Topic</legend>
<br />
<label for='name' >New Subject</label><br/>
<input type="text" id="subject" name="newsubject" value=<?php isse($_POST['newsubject']) ? "{$_POST['newsubject']}" : ""?> /><br />
<br/>
<input type='hidden' name='id' value='<?php echo $id ?>'/>
<input type="submit" name="save" value="Save New Topic" />
</fieldset>
</form>
</div>
<?php
if (isset($_POST['save']))
{
$subject_save = $_POST['newsubject'];
require_once("db_connection.php");
$conn = connectToMySQL();
$id =$_POST['id'];
$query = "UPDATE tbl_topic SET subject = '$subject_save' WHERE id = $id";
$result = mysqli_query($conn, $query)
or die("Error in query: ".mysqli_error($conn));
header("Location: forum.php");
die();
}