I'm a new user of PHP and MySQL, already making progress in a very short time and astonished at how totally cool it all is. As always when I learn new stuff I'm jumping in to the deep end--or at least it seems like the deep end to me.

Basically what I want to do is allow people who have the right to log onto the unix machine to be able to logon to my website with thier unix usernames and passwords and then change their passwords on the site. I'm seeing tutorials etc on authentication/tracking, well and good, but it's usually with a database, cookies, flat files, .htpasswd files etc. These I can manage.

What I would like is some advice on how to do that. How do you use PHP to log onto a UNIX system and change the password via a database? What should I look for? Where do I start? I'm not at all averse to figuring out how to tie things together myself, but it would be nice to hear what to do and what not to do from experienced folks who've done it--things to watch out for etc...