Nikita Popov has a new post to his site looking at how you can add your own syntactic features directly to PHP (requires knowledge of the C language).
Several people have recently asked me where you should start if you want to add some new (syntactic) feature to PHP. As I'm not aware of any existing tutorials on that matter, I'll try to illustrate the whole process in the following. At the same time this is a general introduction to the workings of the Zend Engine. So upfront: I apologize for this overly long post.
He covers the usual "life" of a PHP script, how tokenization is handled and what happens when the script is parsed, compiled and executed. Code snippets are included to show you the points to add in your own syntax item - in their case, adding an "in" operator to see if a value is in an array (a one word version of this).