user (system) authentification

Hi, is there a way, to verify a password via php, if it matches it's entry in /etc/shadow ? I am trying to write a script, which needs a user authentification on a server wide basis and since the users come and go rather fast on the server in question, i thought that the best way would be to let the user enter his system username & password and validate it with the one set in /etc/shadow for that entered username. md5() didn't work, so i guess that i am using the wrong method. Does anyone know, how i can achieve this? Regards, Duncan