Sounds like a pretty solid server. Since it's CentOS, I was just thinking do you happen to have SELinux installed and activated? If so, could you try turning that off (i.e. disabling) and see if it helps? I'm sorry (still in the wild guess zone). With SELinux you have to be very careful with setting the SELinux file/folder permissions but I never use it with my web servers so unfortunately I might not be of much help there.
As for where else could errors be logged, if you create a file showing phpinfo() (see http://php.net/manual/en/function.phpinfo.php) , and call it from a browser, it should show you the current logging location as part of the information displayed.
You could also try manually installing the database. since it sounds like you have root access from the command line.
Check if Tiki has already created a db/local.php. If so, the connection to the database is already setup. You can try the command line command in the tiki root:
php console.php database:install
(try php console.php database:install --help for more info, or see https://doc.tiki.org/Console for more general information about the console)
If local.php has not yet been created, you could try creating it manually first: see https://doc.tiki.org/Manual+Installation.