--- /dev/null
+What do all those files mean?
+=============================
+
+ * `php5.y`: PHP 5 grammar written in a pseudo language
+ * `php7.y`: PHP 7 grammar written in a pseudo language
+ * `tokens.y`: Tokens definition shared between PHP 5 and PHP 7 grammars
+ * `parser.template`: A `kmyacc` parser prototype file for PHP
+ * `tokens.template`: A `kmyacc` prototype file for the `Tokens` class
+ * `rebuildParsers.php`: Preprocesses the grammar and builds the parser using `kmyacc`
+
+.phpy pseudo language
+=====================
+
+The `.y` file is a normal grammer in `kmyacc` (`yacc`) style, with some transformations
+applied to it:
+
+ * Nodes are created using the syntax `Name[..., ...]`. This is transformed into
+ `new Name(..., ..., attributes())`
+ * Some function-like constructs are resolved (see `rebuildParsers.php` for a list)
+
+Building the parser
+===================
+
+In order to rebuild the parser, you need [moriyoshi's fork of kmyacc](https://github.com/moriyoshi/kmyacc-forked).
+After you compiled/installed it, run the `rebuildParsers.php` script.
+
+By default only the `Parser.php` is built. If you want to additionally emit debug symbols and create `y.output`, run the
+script with `--debug`. If you want to retain the preprocessed grammar pass `--keep-tmp-grammar`.