There's JLexPHP: https://github.com/wez/JLexPHP/blob/master/jlex.php
I've not used it, but there's this: http://pear.php.net/package/PHP_ParserGenerator , which creates a PHP Parser from a Lemon grammar. The project seems to be inactive though.
I also found this project: http://code.google.com/p/antlrphpruntime/ , which uses Antlr. Again inactive though.