Antlr is a pretty good parser generator. It targets a number of languages, including Java, Python and C. I've been working on a Php target for Antlr. The code is still in early stages but quite usable. Have a look at the documentation to get started.

There is also a pre built Antlr download to use. The source is available on svn.

svn checkout http://antlrphpruntime.googlecode.com/svn/trunk/ antlrphpruntime

Lots of stuff works, but I still have a long way to go before it is anywhere near complete. So expect issues. The best way to find out what works is to look at the test cases in the svn repository.