This is a standard debian package manager installation and the directory structure must be left as is. In this installation, dokuwiki has already been initialized and you should already have a user named
admin[/m] and you should have been given the opportunity to choose a password for [m]admin[/m]. There is no need to use install.php. However, if you want to install plugins you will have to change the owner of [m]/var/lib/dokuwiki/lib/plugins[/m] from [m]root[/m] to [m]www-data. Moreover, if you ever want to upgrade your dokuwiki using the upgrade plugin, you will have to make the same change throughout the installation.
The debian dokuwiki is at Angua, which is now 3 years old. You might want to consider uninstalling this version and installing the latest version in /var/www. You can do this very easily using the dokuwiki downloader:
https://www.dokuwiki.org/install:downloader
Click on the link to the downloader script provided there and save it to your desktop. Create a directory in /var/www. Change the owner of that directory to
www-data, place the script in that directory and run it from your browser. Do not name this new directory "dokuwiki" unless you have removed the package manager's dokuwiki.