Why is php unsetting session variables on its own?

[dave_15_uk]

When i refresh a page, all of my session data is reset. I havent used unset() or session_destroy() anywhere on the page.

Code
$id = $_SESSION['userid'];
$loggeduser = $_SESSION['user'];
$userfromdb = mysql_fetch_array(mysql_query("SELECT username,id,code FROM users WHERE username='$loggeduser' AND id='$id'"));

database id <?=$userfromdb[id]?><br />
cookie id <?=$id?><br />
username <?=$loggeduser?>

Before refresh -

database id 3
cookie id 3
username Dave

After refresh -

database id
cookie id 3
username Array

pleeeeeeease help !