For an error like this
Notice: A session had already been started – ignoring session_start() in .. on line ..
This happens when you try to start session more than once.
The solution for above problem is
1) in php.ini file set session.autostart to 0
session.auto_start = 0
2) In your code use this line
if (!session_id()) session_start();
instead of
session_start();