AuthPAMOld has a small requirement that must be met in order to work, you need pam_auth php module. I've seen it's on some Linux distributions but not on all.
On the main page AuthPAM Link's section you can get the link to the author's home to get the module and compile it if your distribution doesn't include a binary package.

After compilling and installing the module as stated in it's documentation you have to create a PAM service for Tiki (normally at /etc/pam.d).

Then just jump onto Tikiwiki and setup PAM there.

Take care of file permissions, remember that php runs with apache privileges, normally a normal user account like www-data or so. If you plan to use PAM
against your system's users and your system uses shadowed password you should remember that /etc/shadow is only readable by root, so php won't be able to read it,
(the PAM library runs with the calling user privileges) so you'll have to workaround it, maybe letting your webserver's user read shadow file or so.

I'd be glad of any suggestions on that point

With a default PAM service any account will be granted (try user: nobody 😛 ) so here are a few things to take care about that:

I recomend you to make use of pam_require module to require a specific group to be in for the user. Also you can take a look at PAM Modules at kernel.org to refine a bit more your pam service for tiki.

Also take note that pam only receive a user/pass pair and checks it, it relays on your web server settings to handle a secure transaction of that pair from the browser to the server, I recommend SSL 😛

Thanks damian for advising me that two things.

Who is working here generally? Link UserPage.