a few weeks back this topic had some discussion (cassandra as a session store). subsequently, i threw together a quick hack to have PHP use Cassandra as a session store. A benefit I quickly found is that I could rely on Cassandra to expire the sessions and not PHP session garbage collection. nice.
The code is at the following URL and uses phpcassa as the interface to thrift.
Sessions were given their own column family, with the row key defined as the site name. each column is a session id with the column data being the session data.
Works a treat. Each time PHP session_start() is called, the expiry on the column resets back to the original value. So, if the expiry is set to 1 hour, it's 1 hour after the last session_start() was invoked.