Pressflow Wiki: Using Cassandra with PHP
On the Pressflow wiki there's a handy guide to getting PHP up and working with the Cassandra, the Apache Project's distributed database solution. (Note: Pressflow is Drupal-related, but this tutorial isn't specific to that.)
The guide walks you through the steps you'll need to build the Thrift client (with a list of a few dependencies) and build the PHP extension using this install. With the module built, they help you change the php.ini to load the shared module and include the commands you'll need to start up the Cassandra server. There's also a link to a sample script you can use to test out the full installation.
If you're having trouble making the connection, read through the comments for some common problems from other users.