Error in session declarations.
1)Session is already started and you are trying to start it again
ie, session_start() is called more than once.
2)session.auto_start may be enabled in your php.ini file.
So if you call session_start() again it will throw an error.
You can verify this in php.ini file or run a test file which echo phpinfo().
This is a common error happens to beginners.
1)Apache web server is not started or it is misconfigured
2)Missed to add php tags in the code
3)PHP short tags are not activated in php.ini configuration file