Eva,
Your technical question, in a technical mailing list, is highly inappropriate... you should know better. Skunks-ke is here for ads, offtopic threads and endless discussions on rules ;)
Having gotten that out of my system: LimeSurvey
http://www.limesurvey.org/ seems to fit your bill. In particular, it supports conditional questions (where the questions asked depend on previous answers). It is open-source, and written in PHP, so you can hack it if you need to. There are a gazillion other similar applications, many of them opensource, but LimeSurvey is what I have most experience with.