What do all those files mean?

zend_language_parser.y: Original PHP grammer this parser is based on
zend_language_parser.pre.phpy: PHP grammer written in a pseudo language, which is transformed into a proper kmyacc grammer using preprocessor.php
zend_language_parser.phpy: PHP grammer ready for kmyacc
analyzer.php: Analyzes the .pre.phpy-grammer and outputs some info about it
preprocessor.php: Transforms a .pre.phpy grammar into a .phpy grammar
rebuildParser.php: Builds the actual parser by calling kmyacc
php.kmyacc: A kmyacc parser prototype file for PHP
y.output: kmyaccs debug output

.pre.phpy pseudo language

The .pre.phpy file is a normal grammer in kmyacc (yacc) style, with some transformations applied to it:

Nodes are created using the syntax Name[subNode1: ..., subNode2: ...]. This is transformed into new PHPParser_Node_Name(array('subNode1' => ..., 'subNode2' => ...), #this->line) (# is used instead of $ because $ is a reservered character in kmyacc grammars. It is later transformed back to $)
Name::abc is transformed to PHPParser_Node_Name::abc
Some function-like constructs are resolved (see preprocessor.php for a list)