Laravel 4.2 - Session not working after upgrade to Fedora 26 (PHP 7.1)

My Laravel 4.2 application stopped working after I upgraded from Fedora 25 to Fedora 26. This included an upgrade from PHP 7.0.x to PHP 7.1.x.

It mostly seems fine, but nothing is stored in the session. I was using memcached, but tried out file-based sessions just in case. Viewing the cookies in the browser development tool doesn't seem to reveal a problem with cookies. Login succeeds, attempts to update the session, but the values are not available to the next request.

Does anyone know what I can tweak to get this working again?