A session had already been started – ignoring session_start()

The solution two this error is one of two things, either session create is not checking for an existing session before starting one, or the php.ini has session.auto_start enabled.

Make sure you create a session by checking if one doesn’t already exist:

if(!isset($_SESSION)){
    session_start();
}

If you are sure this has been set properly then the following could be the solution:

// php.ini
session.auto_start = 0

Restart apache / php-fpm and see whether this solves your problem