Errr - that's not right at all. The only reason that you cannot start a session (assuming that PHP version allows for them and PHP is set up correctly) is that you have dumped some output to the browser already before you try starting a session.
The session is set up in the browsers header fields, and so any output must not come before this has started. Even an extra space before your <?php tag will be sent to the browser and stop you from setting up a session.
So - make sure that your session call comes before any output and you should find that things all work fine 🙂