session question

I am setting up a website with a need to use some sort of session management for a large amount of users. I cannot use typical file based session managment because at any given time there could be up to a million users logged in at once. (It is a LAMP linux/apache/php4/mysql system). I am a bit confused though as how to go about this. The user will be authenticated by verifying a username/password combo in a database, and then a session created. My question is this: After authentication, which type of session managment should I use? I mean, just do the standard php stuff with the session_ functions? (wo'nt this be bad with the # of simoltaneous users i need to support, because of the # of files on the server?) Or, shall I use something more complex like PHPLIB or create my own scheme using mysql? Is there any exisiting code/functions that can make creating my own scheme easier in order to support mysql or am i way off with this question? I just need a bit of direction here and any help is appreciated. Thanks! -Derick