Security question with PHP on Unix / Linux.

Hi, 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 in Unix / Linux. Do they get to login into the Unix / Linux system through separate accounts? In that case, 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? The end goal is to separate out uses and to make sure that normal users (who do not have admin rights) do not get to read a file, write into a file etc from areas that they have no access rights into. How is this access rights set in Unix / Linux? Your help in this is appreciated. Thanks, Ananth.