session - help

Hi, Am just a beginner to PHP, jumping from ASP. I tried re-coding from my small ASP site. And the things worked fine until I met the login page. I store the user data in sessions and access then. The logging name, mail-id, userID are stored in session vars. I used this <?php $_SESSION ["MemberName"] = "Whatever"; $_SESSION ["MemberId"] = "Whatever"; $_SESSION ["MemberMail"] = "Whatever"; ?> to store the data on successful login attempt. The page redirects to home page after that. When I try to access the session var there... it gives a notice saying... the session variable is not defined?? How can I avoid that. I didnt do with session_start () or things like that... as we dont do that for in ASP. Thanks, SPS