hi about authentication with php

hi all i will be happy if anyone can help me about this problem "how we can do Form authentication with php " my mean is creating a login page in restricted area of site that only authenticated user can view content of this section and if anyone wants to bypass login form automaticly redirect to login page" if this manner is available how and if not how php sites authenticate users" form authentication is availabe in asp.net & jsp but i dont know wether or not available in php. thanks/