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 p

For this we can use the session objec($_SESSION)t. 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.