Filter server side processing with $_SESSION['user_id']

Hi
Can anyone point me in the direction of a straight forward example, showing how to filter the data returned from "server_processing.php". I want to limit the data returned to match the user_id to whom it belongs to. I have had a good look through this forum, however, a solution seems to evade me.
David
P.S
Because I am still in the stage of figuring out where to start, I am unable to post any code or work in process examples.