replace not updating in database

I have a table with id which is the primary key and user_id which is a foreign key but the session is based on this in my code.
I have tried EVERYTHING, so I will post my full code.
The form should insert if there is not a user_id with the same session_id in the table. If there is, it should update.
At the moment, when the user has not visited the form before (no user_id in the table) and data is inserted in, the page returns to the location page: but the data is not inserted in the table. if the user changes the data once it is updated it doesn't change either.
This is the table structure:

`thesis` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `user_id` int(11) NOT NULL,
   `thesis_Name` varchar(200) NOT NULL,
   `abstract` varchar(200) NOT NULL,
   `complete` int(2) NOT NULL DEFAULT '1',
    PRIMARY KEY (`id`),
     KEY `user_id` (`user_id`)
    )
 

The code I have been using (and failing):

$err = array();
 
 
 $user_id = intval($_SESSION['user_id']);
 // otherwise
 if (isset($_POST['doThesis'])) {
 $link = mysql_connect(DB_HOST, DB_USER, DB_PASS) or die("Couldn't make connection."); 
 // check if current user is banned
 $the_query = sprintf("SELECT COUNT(*) FROM users WHERE `banned` = '0' AND `id` = '%d'",
 $user_id);
 
 $result = mysql_query($the_query, $link);
 $user_check = mysql_num_rows($result);
 // user is ok
 if ($user_check > 0) {
 
     // required field name goes here...
     $required_fields = array('thesis_Name','abstract');
     // check for empty fields
     foreach ($required_fields as $field_name) {
         $value = trim($_POST[$field_name]);
         if (empty($value)) {
             $err[] = "ERROR - The $field_name is a required field" ;
         }
     }   // no errors
     if (empty($err)) {
         $id = mysql_real_escape_string($_POST['id']);
         $thesis_Name = mysql_real_escape_string($_POST['thesis_Name']);
         $abstract = mysql_real_escape_string($_POST['abstract']);
   //replace query
         $query = "REPLACE INTO thesis ( thesis_Name, abstract) VALUES ('$thesis_Name',
        '$abstract') where id='$_SESSION[user_id]'";
         if (!mysql_query($the_query))
             echo "the query failed";
         else header ("location:myaccount.php?id=' . $user_id");
    }}}
 
         $rs_settings = mysql_query("SELECT * from thesis WHERE user_id = $user_id;");
 
 
         ?>
 
 
         <br>
         <form action="thesis.php" method="post" name="regForm" id="regForm" >
             class="forms">
 <?php
 $num_rows = mysql_num_rows($rs_settings);
     if($num_rows > 0) { ?>
 
 <?php while ($row_settings = mysql_fetch_array($rs_settings)) {?>
  Title of Proposed Thesis<span class="required">*</span>
  <textarea name="thesis_Name" type="text" style="width:500px; height:150px" 
 id="thesis_Name" size="600"><?php echo $row_settings['thesis_Name']; ?> </textarea>
                 </tr>
 
                 <tr>
                     <td>Abstract<span class="required">*</span>
                     </td>
                     <td><textarea name="abstract" style="width:500px; height:150px"  
  type="text" id="abstract"  size="600"><?php echo $row_settings['abstract']; ?>  
   </textarea></td>
                 </tr>
 
 
                 <?php }
                 } else { ?>
 
 //shows fields again without echo
 

I've tried var_dum($query) but nothing appears

PS I know the code isn't perfect but I'm not asking about this right now

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