Hello and welcome to LinuxQuestions,
I use Hearbeat in combination with ldirectord (I know Pacemaker is the new toy in town but haven't come around to testing or installing it). We don't even come close to a million customers on a daily basis but are more in the range of a couple of thousand users per day. I have our configuration set so that a session will timeout after 5 minutes of inactivity so that users always stay connected to the same server. That means that when one of our users logs in he/she stays connected to the same server for the duration of his session OR untill there was a period of 5 minutes of inactivity.
We also use a 'LAMP' application as you call it, connected to a NAS for document storage and MySQL databases and I didn't have to do any change in configuration of the application, only on host level for what concerns the Heartbeat and load balancer configuration and the virtual IP configurations.
You can read more about Heartbeat and Pacemaker (the site also has links to other interesting software) on this site:
Linux HA
Kind regard,
Eric