Apache
Running a redundant apache server does not require much from apache itself, but rather the underlying filesystem and a proxy in front of it. The filesystem is important, so the website and its php sessions can be share between the servers. For this we will use GlusterFS. The proxy in front of apache will be a combination of HA-proxy and Corosync. Another backend apache depends on is the Mariadb server.
In this setup we will configure 2 apache servers.
Software
apt-get install apache2 php php-mysql