Security question with PHP on Unix / Linux.

Hi, I am working on porting PHP onto NetWare. At this point of time, I am trying to understand the way security is implemented for PHP on Unix / Linux. I mean, how are the different users distinguished from Unix / Linux. Do they get to login into the Unix / Linux system? Do they have separate data space for each user? What is the API that is used to login to Unix / Linux. User may enter his username and password on the browser, but how do they get translated onto the Unix / Linux box? Your help in this is appreciated. Thanks, Ananth.