ALL Linux distributions are compatible with every piece of software for every other one. Distributions are just packaged deals, sometimes with different methods for installing programs, but in the end, they are all exactly the same.
They are almost all totally free and available for download.
Oh and they do PHP and almost all web stuff, as Linux runs like 70% of webservers on the internet.
Here's a few links to some good distributions:
www.debian.org
... JakeC, finish off my list please. :classic: