Ask HN: Good Open Source Survey Software?

I was wondering if anyone could recommend a good open source survey system, preferably in PHP.

I'd like one with a bit more intelligence than just counting up the answers, for example:

- For a question of "which of the following sites do you read (tick all that apply)", it would be nice to also be able to see statistics such as "x% read more than 3 of them" and "of people who read site A, x% also read site B"

- Tracking between answers: show results from question A made up only of people who also answered x in question D, etc.

Or would I be better with a paid hosted service?

Thanks