Remove use of DIRECTORY_SEPARATOR
Currently a lot of places use the PHP constant DIRECTORY_SEPARATOR. This constant belongs to the directory functions (see http://de.php.net/manual/en/ref.dir.php), i.e. it is not even defined for the usual filesystem functions like fopen nor is it required for include/require. PHP translates a forward slash automagically on Windows.
When specifying paths in php.ini it is even required to always use a forward slash, even when on Windows.
What I want to say is: let's remove this ugly thing and just use / everywhere.
For the sake of completeness: when calling local commands with exec() and friends one has to use the "right" separator, this is when it might come in handy...