Refactor directory access code

In some places it's used with `DIRECTORY_SEPARATOR` in other places it is not.

We should decide for either one of those methods.

The constant is actually not needed, so I say we throw it out, see: http://www.php.net/manual/en/dir.constants.php#113236

Or we at least use this method in all places: http://www.php.net/manual/en/dir.constants.php#113343
