The manual page about session_register (http://www.php.net/manual/en/function.session-register.php) says:

"Caution: If you are using $_SESSION (or $HTTP_SESSION_VARS), do not use session_register(), session_is_registered() and session_unregister()."

and a few lines later

"If session_start() was not called before this function is called, an implicit call to session_start() with no parameters will be made."

But the latter is not true if only $_SESSION is used to register a session variable. The current implementation requires a manual call to session_start().

This should be noted there.

Idially, it should be mentioned also at http://www.php.net/manual/en/ref.session.php (where it says: Note: As of PHP 4.1.0, $_SESSION is available as global variable just like  ...) and maybe also at http://www.php.net/manual/en//language.variables.predefined.php (which may be too much good will though ;)