In attempting to write cross-platform, portable PHP engine, I was using PHP's DIRECTORY_SEPARATOR constant to write path strings(eg ".." . DIRECTORY_SEPARATOR . "foo.php", because the proper way to write it in Windows would be "\" while in Unix it would be "/".

Well, an engineer from php.net pointed out to me that, and a few other programmers confirmed that using that constant is completely unnecessary. As long as you use the forward slash (/), you'll be fine. Window's doesn't mind it, and it's the best for *nix operating systems.

(Note that DIRECTORY_SEPARATOR is still useful for things like exploding a path that the system gave you.)

Hope this is useful to people.