Sessions in object oriented code

Hi, Hope I can explain this as easily as possible, basically I am using both cookies and sessions for my script, whereby the user is allowed to choose which method they want to login with. Problem for me is removing the registration form, etc., from those that are logged in. The thing is the form is in its own method in a seperate file, and its called within HTML code so obviously if I included session_start() in the seperate include file where the methods/classes are, etc., I'd get a "headers already sent" error. So is there a solution to this? Thanks.