This expression – dirname(__FILE__) – is used in a real lot of places. The reason is simple – libraries want to include files relative to library top directory, and do not want to count on include path. And relative include resolution rules in PHP not clear to all, so people prefer to be sure. The downside here is that this expression is dynamic – executed at run-time. Meaning it’s slower and less toolable and also makes a bad habit of putting dynamic things into include (which is not a problem here, since it’s “static dynamic” thing, but still a bad habit).
So why won’t we have constant that would mean dirname(__FILE__)? Something like __FILEDIR__. Would make a lot of code cleaner.
The problem with it of course to make real use in code we should travel back in time to 1996 and add it there :)