Linux noob here. I found the php files, but I don't know how to open or view them.
If you have a SFTP client just point it to your ip with minera/minera as user/pass you should be able to navigate the filesystem and get the files (then just open them with a text-edit)
Files are in /var/log/minera or /var/www/minera/application/logs (it's a directory link)