Session variables not persistent in PHP5/Apache2/Ubuntu12
Solution 1:
You said that /var/lib/php5/session is owned by root and has permissions drwx-wx-wt. This means that all users can write to the directory, but only root can read. The result is that PHP can create session files, but cannot go back and read them later. This appears to be why your sessions are failing.
Fix the ownership and permissions so that www-data can both read and write (rwx). For instance:
chgrp www-data /var/lib/php5/session
chmod g+rwx /var/lib/php5/session