"Does PHP replicate sessions between those two linux boxes?"
You're missing the point. It's not PHP's job to replicate sessions (PHP doesn't re-invent a whole bunch of existing technologies). [of course there's an expection - PHP's MSession extension]
PHP simply provides a session API to store sessions _somewhere_ - it's up to you where you put the sessions - in the filesystem, in a database, in shared memory.
Once stored, replication happens at a lower level - filesystem / network / OS and Linux does a very good job of clustering these days.
Doing things this way (as opposed to using an application server) fits nicely with typical corporate infrastructures - most importantly the existing sysadmin staff will know how handle it (vs. having to become a Java Sysadmin as well).