cant log out...

[error_22]

I have a simple log in/log out script, but for some reason I can't log out. Here's the code:

PHP Code:

if( isset( $_GET['action'] ) )
{
    switch( $_GET['action'] )
    {
        case 'login':
            include( './auth/data.php' ); //the login script
            break;
            
        case 'logout':
            session_destroy();
            header( 'location: ./index.php' );
            break;
    }
}

session_start();

if (!isset($_SESSION['u_id'])) {

echo "you are not logged in! <br />";
echo '<form action="index.php?action=login" method="post">';
echo '<input type="text" name="username" />';
echo '<input type="password" name="password" />';
echo '<input type="submit" value="Login" /></form>';
}

else {

echo "you are logged in!";
echo '<a href="index.php?action=logout">Log Out</a>';
} 


So when I hit the "Log Out" link, the page reloads, but I'm still logged in. What am I doing wrong?

Thanks in advance
Niklas