Quick session var ?

I am just starting to learn sessions in PHP and have a quick question (yes I have read all of the relevent docs but am still confused on several points). My goal is to write a script that is usable in newer and older vers of PHP wheteher register_globals is on or off. session_register() puts a $var in the session and if register_globals=on it will be extracted and available as $var on the next page after session_start? if register_globals=off it will be available via $HTTP_SESSION_VARS? With register_globals=off on newer PHP versions I should use $_SESSION to set and get session $vars? With register_globals=on the $vars from $_SESSION are extracted? Any pointers on what would be a flexible way to handle sessions regardless of PHP version or register_globals would be very helpfull. Thanks! Shawn