Sessions not working on php 7.1.3

Ubuntu 16.10 + PHP from a custom repository:

Code:

ivan@ivan:~$ php -v
PHP 7.1.3-2+deb.sury.org~yakkety+1 (cli) (built: Mar 15 2017 09:23:43) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.1.3-2+deb.sury.org~yakkety+1, Copyright (c) 1999-2017, by Zend Technologies

I don't experience problems with sessions. I use the "files" driver.