Changing Password Script - Please Help!!!

[Refon Miah]

Hi. I am using PHP5 and MySQL5, running on Apache Server on Windows XP.
I am trying to use the scripts below to change a user's password, but i get the message saying 'Password Could not be changed' (Line 15 in the second script).

(change_passwd.php)
=========================================================
<?php
require_once('manormotors_fns.php');
session_start();
do_html_header('Changing password');

$old_passwd = $_POST['old_passwd'];
$new_passwd = $_POST['new_passwd'];
$new_passwd2 = $_POST['new_passwd2'];

try
{
check_valid_user();
if (!filled_out($_POST))
throw new Exception('You have not filled out the form completely.'
.' Please try again.');
if ($new_passwd!=$new_passwd2)
throw new Exception('Passwords entered were not the same. Not changed.');
if (strlen($new_passwd)<6)
throw new Exception('New password must be at least 6 characters.'
.' Try again.');
// attempt update
change_password($_SESSION['valid_user'], $old_passwd, $new_passwd);
echo 'Password changed.';
}

catch (Exception $e)
{
echo $e->getMessage();
}
display_user_menu();
do_html_footer();
?>
===========================================================

and

change_passwd function (in user_auth_fns.php)
===========================================================
function change_password($username, $old_password, $new_password)
// change password for username/old_password to new_password
// return true or false

{
// if the old password is right
// change their password to new_password and return true
// else throw an exception
login($username, $old_password);
$conn = db_connect();
$result = $conn->query( "update user
set passwd = '$new_password'
where username = '$username'");
if (!$result)
throw new Exception('Password could not be changed.');
else
return true; // changed successfully
}
============================================================

Please help me!!! I am Really stuck!