Update a variable on click - php

Question

basically I've written the following but I'm wondering what the best way to get $message_id = to the $row['id'] you click on to view an inbox message. If I just statically set $message_id = to say 39 and click on the row that's $row['id'] is 39 it'll open the message.
Basically what it does is list inbox messages I have, and anchor tag them with its row id to be used in the header to show the full contents of that message.
<p>
<?php
$message_id = "39";
if (isset($_GET[$message_id]) && empty($_GET[$message_id])) {
$username = $user_data['username'];
$get_message = mysql_query("SELECT * FROM `users_inbox` WHERE `id` = '$message_id' AND `send_to` = '$username'");
$get_message_result = mysql_fetch_array($get_message);
echo '<h1><u>' . $get_message_result['subject'] . '</u></h1>';
echo $get_message_result['sent_from'] . '<br />';
echo $get_message_result['body'] . '<br />';
echo 'Back';
}
$username = $user_data['username'];
$result = mysql_query("SELECT * FROM `users_inbox` WHERE `send_to` = '$username'");
?>
<table border="1">
<tr>
<td>From</td>
<td>Subject</td>
<td>Date</td>
</tr>
<?php
while($row = mysql_fetch_array($result))
{
echo
'<tr>' .
'<td>' . $row['sent_from'] . '</td>' .
'<td>' . $row['subject'] . '</td>' .
'<td>' . $row['post_date'] . '</td>' .
'</tr>';
}
?>
</table>
</p>
Anyone know if this is even possible, as I know code is read from top to bottom and as far as I'm aware you cant go back upwards at all.

Answer 1

I assume that the $user_data['username'] is set or available,if yes then you could pass a variable name of you choice which points the message id instead of sending only the id (eg: i am setting it as msg_id ), Now you can access the message id via $_GET['msg_id']
<p>
<?php
$username = $user_data['username'];
if ( isset( $_GET['msg_id'] ) ) {
$get_message = mysql_query("SELECT * FROM `users_inbox` WHERE `id` = '$message_id' AND `send_to` = '$username'");
$get_message_result = mysql_fetch_array($get_message);
echo '<h1><u>' . $get_message_result['subject'] . '</u></h1>';
echo $get_message_result['sent_from'] . '<br />';
echo $get_message_result['body'] . '<br />';
echo 'Back';
}else{
$result = mysql_query("SELECT * FROM `users_inbox` WHERE `send_to` = '$username'");
?>
<table border="1">
<tr>
<td>From</td>
<td>Subject</td>
<td>Date</td>
</tr>
<?php
while($row = mysql_fetch_array($result))
{
echo
'<tr>' .
'<td>' . $row['sent_from'] . '</td>' .
'<td>' . $row['subject'] . '</td>' .
'<td>' . $row['post_date'] . '</td>' .
'</tr>';
}
?>
</table>
</p>