Typically this is done via PHP session-handling (e.g. starting with the session_start() function, and when the user logs in, add a setting in $_SESSION to indicate whether they are an admin user. Then on your admin-only pages, your logic would be something like:
<?php // this must be the first line: no output of any kind before it
// rest of page...
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation