使用getUser（）的Facebook php sdk登录问题

Question

I have a php script using Facebooks sdk which logins using the following code:

$user = $facebook->getUser();


if ($user) {
$logoutUrl = $facebook->getLogoutUrl(array( 'next' =>      ($user['baseurl'].'logout.php') ));
} else {
$loginUrl = $facebook->getLoginUrl(array('scope'=>'offline_access, manage_pages, publish_stream'));
}

if ($user) {
  echo '<div class="login"><a href="'.$logoutUrl.'">Logout</a></div>';
}
else {
  echo '<div class="login">
    Login using OAuth 2.0 handled by the PHP SDK:
    <a href="'.$loginUrl.'">Login with Facebook</a>
  </div>';
}

The script refreshes and posts to my facebook wall once an hour for a total of nine hours. The problem is that randomly getUser() will fail and I will be forced to manually login again. I'm not sure what's causing the getUser() failure or how to fix it?