What is the default location of session files on an installation of Apache/PHP on Ubuntu 10.10?
First check the value of session.save_path using ini_get('session.save_path') or phpinfo(). If that is non-empty, then it will show where the session files are saved. In many scenarios it is empty by default, in which case read on:
On Ubuntu or Debian machines, if session.save_path is not set, then session files are saved in /var/lib/php5.
On RHEL and CentOS systems, if session.save_path is not set, session files will be saved in /var/lib/php/session
I think that if you compile PHP from source, then when session.save_path is not set, session files will be saved in /tmp (I have not tested this myself though).