One of the simple way is to use session.
session is initialise at top of login script.
then when the user verified his status, you can set a session varible eg. $_SESSION['userid']=userid
then at the top of every php page, you include a check to determine whether $_SESSION['userid'] is valid if it is not valid, redirect to unauthorise login.
of cos there are other ways too. this is just one of the simple way.