Alvaro Videla has a different sort of suggestion for storing the sessions for your application - use Erlang.
In the last few days I been playing with the PHP extension mypeb which allows us to connect to Erlang from PHP. As a simple example to show what we can do with this extension I will create a PHP class that will be used as the session_save_hanlder for PHP. [...] I will like to try something different by using this class to interact with an Erlang node that will act as the in memory storage for our sessions using ETS tables.
He uses the session_set_save_handler PHP function to point the application to a new location (away from the default filesystem handling) and to a custom ErlangSessionHandler object's "open" method. Also included are "close", "read" and "write" methods that let PHP talk directly to the Erlang instance. Code snippets are included.