In this new blog post today, Lars Strojny talks about a new magic constant that will be joining its brothers in the upcoming PHP 5.3 release - __DIR__.
In PHP 5.3 there will be another magic constant __DIR__. [...] To allow this, the internal function php_dirname() has been moved in the Zend Engine and is now called zend_dirname(). Nevertheless an alias still exists.
__DIR__ will join the other constants (like __LINE__ and __FUNCTION__) to help give the currently running script a little introspection for things like its filename, what class it's currently in and now, what directory the file currently lives in.