When I try to start session, I get message that folder '/tmp' does not exist.
I tryed to create one, but wherever I put it error remain. I don't want to change my code, just tell e how can I enable session_start to work fine.

In short, you have to adjust php settings. Run phpinfo(), locate php.ini path, open it in your favorite editor and update path where php keeps sessions session.save_path. You can enter any valid local path there.