A Slim application does not presume anything about sessions. If you prefer to use a PHP session, you must configure and start a native PHP session with session_start() before you instantiate the Slim application.

You should also disable PHP’s session cache limiter so that PHP does not send conflicting cache expiration headers with the HTTP response. You can disable PHP’s session cache limiter with:

<?php
session_cache_limiter(false);
session_start();