At the moment Phalcon does not support Cassandra PDO - actually you are the first person to ever ask for it :)
You can however implement it yourself using these guides
http://docs.phalconphp.com/en/latest/reference/db.html#implementing-your-own-adapters http://docs.phalconphp.com/en/latest/reference/db.html#implementing-your-own-dialects
Sample implementations can be found in our incubator repo here:
https://github.com/phalcon/incubator/tree/master/Library/Phalcon/Db/Adapter