Been for looking for this kind of thing for a while. After finding this post, I've tried the ANTLR PHP runtime. I can report that it's far from being finished. There are several errors in the generated code, where the original java runtime classes has not been properly translated to PHP (nested class declarations, using '.' instead of '.' when trying to access class methods operator).
The ANTLR framework itself is quite powerful (can't attest to the efficiency of the generated code).
Especially the graphical tool ANTLRWorks makes it easy to create and debug grammas. Just too bad about the PHP version. It's possible to roll your own though. The best solution may be to analyse the generated ANTLR runtime class, figure out how it's works, and come up with a light weight less enterprisey version thereof.