On this video (at 29:00), Eric Evans from Rackspace tells an audience that using Thrift and Avro is a bad idea. Instead, he advocates using the Idiomatic Client Libraries.
http://video.disruptivecode.com/video/840645/what-makes-cassandra-trick
We're primarily coding in PHP. How do we connect PHP with Cassandra without Thrift or Avro?
The Idiomatic Client Libraries that Eric mentions are:
Cassandra High Level Clients (The Cassandra wiki is a great place on earth)
In your case I would recommend Tylers Hobb's phpcassa.