Keeping session (userID) the same whilst changing pages - php

Question

My users have a user_ID when logging in and this appears within the URL. However when moving to a page from 'myaccount.php' when the next form has been saved (thesis.php) the same ID should still be in the URL and therefore the correct data they inputted should be under their own ID in the database. At the moment the thesis.php page just refreshes with the following code:
$err = array();
$result = mysql_query("SELECT `id` FROM `users` WHERE `banned` = '0' ORDER BY
`id` DESC");
if(isset($_SESSION['user_id']))
if(empty($_SESSION['$user_id'])) { // user not logged in; redirect to somewhere else }
if (!empty($_POST['doThesis']) && $_POST['doThesis'] == 'Save')
{
list($id) = mysql_fetch_row($result);
session_start();
// this sets variables in the session
$_SESSION['user_id'] = $id;
foreach($_POST as $key => $value)
$stamp = time();
$ckey = GenKey();
mysql_query("update users set `ctime`='$stamp', `ckey` = '$ckey' where id='$id'")
or die(mysql_error());
$row = mysql_fetch_array($result);
print_r($row);
if(empty($err)) {
$thesis_Name = mysql_real_escape_string($_POST['thesis_Name']);
$abstract = mysql_real_escape_string($_POST['abstract']);
$sql_insert = "INSERT into `thesis`
(`user_id`,`thesis_Name`,`abstract` )
VALUES
('$id','$thesis_Name','$abstract') ";
mysql_query($sql_insert,$link) or die("Insertion Failed:" . mysql_error());
}
header("Location: myaccount.php?id=' . $_SESSION[user_id] .'");
exit();
}
}
HTML:
<p align="center">
<input name="doThesis" type="submit" id="doThesis" value="Save">
</p>

Answer 1

I believe you need to call session_start() before you can do isset($_SESSION['blah']).
Try shifting session_start(); to the top of the file
<?php
session_start();
.....
the rest of your code

Answer 2

session_start(); Must be at the top;
if you want to pass the id var in the URL use
<a href='myaccount.php?id=<? echo $id; ?>'>