I'm not sure what you mean, but once you create a session variable, that variable is always accessable until you close the browser.
For example, if you create the variable $logged_in
if(!session_registered("logged_in"))
session_register("logged_in);
$logged in = 1;
then you go over to your next script which contains:
if(!session_registered("logged_in"))
session_register("logged_in);
then if you
echo $logged_in;
the output will be 1
Hope this helps.
Refer to the PHP manual on sessions for more information.