Session variables not working php

1. Make sure session_start(); is called before any sessions are being called. So a safe bet would be to put it at the beginning of your page, immediately after the opening <?php tag before anything else. Also ensure there are no whitespaces/tabs before the opening <?php tag.
2. After the header redirect, end the current script using exit(); (Others have also suggested session_write_close(); and session_regenerate_id(true), you can try those as well, but I'd use exit();).
3. Make sure cookies are enabled in the browser you are using to test it on.
4. Ensure register_globals is off, you can check this on the php.ini file and also using phpinfo(). Refer to this as to how to turn it off.
5. Make sure you didn't delete or empty the session.
6. Make sure the key in your $_SESSION superglobal array is not overwritten anywhere.
7. Make sure you redirect to the same domain. So redirecting from a www.yourdomain.com to yourdomain.com doesn't carry the session forward.
8. Make sure your file extension is .php (it happens!).

PHP session lost after redirect

I had the same issue for a while and had a very hard time figuring it out. My problem was that I had the site working for a while with the sessions working right, and then all of the sudden everything broke.

Apparently, your session_save_path(), for me it was /var/lib/php5/, needs to have correct permissions (the user running php, eg www-data needs write access to the directory). I accidentally changed it, breaking sessions completely.

Run sudo chmod -R 700 /var/lib/php5/ and then sudo chown -R www-data /var/lib/php5/ so that the php user has access to the folder.