One may find empty files named '_dummy' inside some directories (e.g. in ./data/tmp) and wonder what they are good for.

Some directories of DokuWiki are empty at installation time, but still must exist. When DokuWiki is packed into an archive these directories are packed into as well. But, on unpacking, some archive software may omitt to create them because they are empty. The '_dummy' files forces them to always be unpacked (created).

So it's safe to delete them after unpacking.