So you have a cluster of two or more apache servers and want your php sessions to be shared between the different apache servers?
One great solution is to use memcache. It's a Free & open source, high-performance, distributed memory object caching system and can support the mirroring of sessions to multiple memcache servers (which could be on each apache server). Also your sessions are stored in RAM so they are nice and quick too!
This blog post has a nice setup guide. php session clustering with memcache