Sessions Problem with PHP-FPM on Nginx-Collection of common programming errors

Hi everyone I got some trouble with sessions that I use on my little project:

PHP Notice:  Undefined index: user_ID in ../config/sessions_class.php on line 29

(this is from the nginx error.log)

If tried setting the save path for sessions to memcache and to /var/lib/php5/sessions. Session files actually get created in the sessions directory, so the user permissions must be correct. I’m currently saving user settings to sessions such as $_SESSION[‘user_ID’] on index.php and then redirecting to anotherpage.php. If I try to retrieve $_SESSION[‘user_ID’] there I get the undefined index error. session_start gets called several times, but this shouldnt be the problem I guess.

Any suggestions where the problem might be? I can append nginx.conf or php.ini files if you wish.

EDIT: I got Suhosin PHP Hardening – any chance this might interfere with the global SESSION variables?