Best authentication method for user

Hi, I would like to use a secure way how my user is authenticated. For that i was thinking to use session and register his login and password as session variable. Like that, when he will need to execute a query he will not have to type again everything but application will do it alone. What do you think about that ? Moreover, it should exist a default user with which, everybody will be able to execute some basic queries. i was thinking to store this login and password in hardcoding in some text file that will be used at the first web page (some file that i will include in my PHP code).... after everything will be managed by PHP sessions and variables. i know that hardcoding is not the best secured solution, but really default user should not enter his login and password. So how can i do that ? thanks a lot for your answers. Alain