I would like to clarify one thing. When you say that PHP allows people to build interactive so-called Web 2.0 websites, it is kind of like saying, that the use of "red cars" allows people to drive on roads.
Every server side language, that has the capability to evaluate parameters of http requests (user input) and to create specific output based on this input, can be used to create Web 2.0 applications/websites.
Web 2.0 has to be seen as a concept, and has nothing to do with a specific server side language.