session.gc_maxlifetime should be a relative value.

Without the addition of time(NULL), any value greater than 2678400 (30 days)
is treated as absolute.  A 31 day gc_maxlifetime would result in empty sessions
because the memcached data is immediately timed out.

introduced in commit php-memcached-dev/php-memcached@39d0beed1cc72947a8cac71ed25fd9d2deb3c274
fixes #142