PHP Update query not receiving variable already defined in page - php

Question

Hi I'm trying to update a single field from a HTML form, for some reason one of the session variables I am passing to the update query is not being accepted. I have already echoed the variable in the page so am fairly certain it exists in memory.
NB, I know my code is horrifically insecure but I'm learning PHP and once I've got the basics working Ill go over it and bring it upto best practice standards.
E2A: If I do var_dump($filename); before trying to run the query it returns string(6) "356/18", after the query it returns NULL. I'm not unsetting the variable anywhere so where could it be going!
Here is my form:
<form method="post" action="">
<p>Your username is: <?php echo $_SESSION['userid'] ?> Your company ID is: <?php echo $companyid['id']?></p>
<h3>Please enter note for file: <?php echo $filename; ?></h3>
<table width="200" cellpadding="5">
<tr>
<th width="18%" align="right" nowrap>Add Note: </th>
<td width="82%" nowrap>
<input type="text" name="note" />
</td>
</tr>
<tr>
<td colspan="2" width="100%" nowrap>
<input type="submit" value="Submit" name="Submit" />
</td>
</tr>
</table>
</form>
Here is my UPDATE query:
$sql = "UPDATE fields SET Notes = ('".mysql_real_escape_string(stripslashes($_REQUEST['note']))."')
WHERE companyId='".$companyid['id']."' AND fileNumber ='".$filename."'";
if($result = mysql_query($sql)) {
echo "<h1>Thank you</h1>Your information has been entered into our database<br><br>";
echo $sql;
echo $filename;
} else {
echo "ERROR: ".mysql_error();
}
} else {
echoing $sql produces the following:
UPDATE fields SET Notes = ('asdasda') WHERE companyId='11' AND fileNumber =''
and here is the bit where I instantiate the POST vars.
include "header.php";
$checkFiles = "checkFiles.php";
// Catches form input from previous page and stores it into session variable called filename for future reference;
$_SESSION['filename']=$_POST['filename'];
$filename = $_SESSION['filename'];
//User id stuff from previous page too;
$userid = $_SESSION['userid'];
$id = mysql_query("SELECT id FROM users WHERE DXNumber='".$userid."'");
// Returns pointer so fetch it as an array and insert it into variable $companyid for later use;
$companyid = mysql_fetch_array($id);

Answer 1

You need to include session_start() on the top of each file.
Just do:
AND fileNumber ='".$_SESSION[filename]."'";
In your update query.
If that doesn't work, make sure that a value for $_SESSION[filename] is being set.

Answer 2

<h3>Please enter note for file: <?php echo $filename; ?></h3>
Create a input box
<input type="text" name="filename" value="<?php echo $filename; ?>"/>
Then filename value will be pass to $_POST array