The SitePoint PHP blog has a tutorial they recently posted from Christopher Pitt covering creating compilers in PHP and examples of it in use.

Many developers avoid writing their own compilers or interpreters, thinking that the topic is too complex or difficult to explore properly. I used to feel like that too. Compilers can be difficult to make well, and the topic can be incredibly complex and difficult. But, that doesn’t mean you can’t make a compiler.

[...] Compilers (and interpreters) begin with humble string manipulation and temporary variables. When they’re sufficiently popular (or sufficiently slow) then the experts can step in; to replace the string manipulation and temporary variables with unicorn tears and cynicism. [...] In our case, we want to maintain most of the PHP syntax, but we also want to add our own little bit of syntax on top. We could create a whole new interpreter…or we could preprocess the new syntax, compiling it to syntactically valid PHP code.

He starts off with some of the basic concepts involved with creating compilers and breaking the current code down into tokens. He shares all the code you'll need to follow along as he goes through the parsing and splitting of the tokens. He shows how to organize the tokens into a more AST-like structure and building it out into a more correct PHP structure. He ends the tutorial by taking what he's helped you create and integrate it with the Pre PHP-based precompiler as a custom compiler. He includes some examples of it in use and a simple screencast of what the end result looks like (a basic task list application).