Aside from what
sde mentioned (and it doesn't appear that you have session_start() at all in your script, else the error message would have mentioned it), both session_register() and session_is_registered() are deprecated and you should instead use the
superglobal $_SESSION. I would guess that the book you're using is outdated.
Please read the
PHP Manual sections on
Session handling functions, specifically the pages for
session_register() and
session_start().