compiler in php..

I'm trying to write a simple compiler in php. Actualy it will be more akin to C's preprocessor with similar syntax. I've been trying to find a php version of Yacc or other tools to aid in making such a compiler *in* php, but I can't find anything. I figure I will need a basic tree datastructure class to put the parsed commands into, but I can't find one suitable (the pear tree class seems like overkill as I dont need to save or load the tree data from xml or a db). Anybody got any ideas on how to make a simple C-like preprocessor for php, written in php?