how to allow 1 user to view an php page

What you are trying to do is not really possible to do with just plain convensional php. The problem is that php-code is run on the server just before data is sent to the user - so you cannot check if someone is on a page in real-time.

Another problem is also what you define as "someone viewing the page". For example, if I load the page and then just leave it there for 24 hours without doing anything, does that still count as viewing it?

So unless you absolutely have to have a code that checks this in real-time, you can do this quite easily. I would add in the database the date and time that your page was last accessed, then, when someone tries to open the page compare the current time with the time the page was last accessed. If the the difference in time is X minutes or more, allow the page to be viewed - otherwise don't allow.

Here is a pseudo-code explaining what I would do:

<?php
  $lastAccess = get_last_accesstime_from_db();
  $currentTime = time();
  if ($currentTime - $lastAccess <= XXXX) {
      //don't allow page to be viewed
  } else {
     //show page
  }
?>

Also, if you want, you could add some javascript to redirect the user to another page once the X minutes of "activity" has ran out. Almost like a "kick" feature.

Though, I really can't imagine why you would want something like this. Perhaps you are trying to do something that can be approached in a better fashion? Do you want to explain why you want only 1 user to view the page at a time?

Best of luck.