manuals manuals...
http://www.php.net/manual/en/ref.session.php
"The second method embeds the session id directly into URLs.
PHP is capable of doing this transparently when compiled with --enable-trans-sid. If you enable this option, relative URIs will be changed to contain the session id automatically. Alternatively, you can use the constant SID which is defined, if the client did not send the appropriate cookie. SID is either of the form session_name=session_id or is an empty string."