sharing variables

hi i'm currently programming a php web application and i have the following problem: i need a login scheme; programmed a simple one. Whichever page the user reaches, in some i need their userid, for extra validation. so basically, i'm looking for a mechanism to share a variable between multiple pages, but as i don't always use forms, i think i can't use url-encoding with the a-tag to make the userid available for all scripts. maybe there is support for logging users in, i don't know anyone an idea? tx