that is rather odd... if this doesn't work you have to have incorrect data in the DB or coming in the post vars... i would of thought.
try using echo var_dump(); as it dumps the data type
<?php
session_start();
require('config.php');
$username = $_POST['user'];
$password = $_POST['password'];
$username = stripslashes($username);
$password = md5($password);
$userdata = mysql_fetch_assoc(mysql_query("SELECT * FROM users WHERE username = '$username' AND password = '$password' LIMIT 1"));
if($userdata['username'] == $username){
$_SESSION['loggedin'] = "true";
$_SESSION['user'] = $userdata['username'];
header("Location: index.php");
exit;
} else {
$_SESSION['loggedin'] = "false";
header("Location: error.php?code=2");
exit;
}
?>