I am not 100% sure this is an Apache issue, however, ever since we upgraded to apache2.4 and PHP7.1 we have been experiencing seemingly random session loss.
No errors are reported and the only data that I've been able to collect so far is that in PHP the $_COOKIE variable will be NULL instead of the array containing the PHPSESSID value.
The result is, all users are logged out and cannot log back in until we reset Apache.
I've tried all the php.ini tweaking I can, and the problem remains. This was not an issue with PHP5.2 and Apache2.2.
I made this ServerFault post which has a lot more detail.
https://serverfault.com/questions/869781/apache-on-windows-cookie-w-php-sessions-bugHopefully someone has an idea, I would hate to have to downgrade =[