Check that user logged in or not

How i will check that user is, logged in or not. i want to make it a function and i want to use in each page and after login i want to go in current page(same page. where i was working).

can anybody tell me how can i do that..?

For this we can use the session object ($_SESSION). When the user login with his/ her user name and password, usually we check those to ensure for correctness. If that user name and password are valid one then we can store that user name in a session and then we can very that session variable has been set or not in a single files and we can include that file in all pages.

You can also use a header based approach, i.e. have a PHP include file which does necessary checking on user logged in or not. Include this file in other files.