MY_controller problem

[eluser]Near[/eluser]
Hi! I'm creating a program which allows user to register and log in to a specific page. This is my problem:

1) validates user if registered or not
2) determine if the user is currently logged in

i managed to finish the first issue.
but the second issue. I created a my_controller which extends the ci_controller.
then i declare a global variable called $is_login.. I can set its value to true on one page. But when i load another page. The value of the variable $is_login was set to false.. Am i doing it wrong?do i have to store the variable to session instead of using my_controller? Thanks!