Kevin van Zonneveld has a new post to his blog revising an older post talking about session management with PHP and how limit the resources needed by them. In this post he points out another method - holding the sessions in RAM rather than on disk.
sing 2007 article Create turbocharged storage using tmpfs, we can defeat some of this over-engineering and take a simpler approach to speeding up sessions in PHP. We'll store them decentralized in memory by mounting RAM onto the existing /var/lib/php5 session directories throughout your application servers, which I will call nodes from now on.
He includes the commands you'll need to make a directory live in the RAM of your machine and how to migrate the existing sessions to this new data store. He covers some of the advantages of this approach (including that there's "less tech" involved so it's easier to manage). There is one point of failure he points out - that it wouldn't be a solution you could use for websites that might need to bridge sessions across machines.