Prevent users from typing the url manually

I'm a php noob and I wanted to ask if you guys know how to prevent users from typing a URL manually? The page isn't ready yet but it might be site.com/custom.php or something like that. To get to the page they need to type in a password which is relatively easy to do in PHP.

Advanced thanks to everyone.

Cheers,
John

 

If it's password protected you should just start a session after validating that the password is correct.

session_start();
$_SESSION['logged_in'] = true;

Then, on the protected page:

session_start();
if (empty($_SESSION['logged_in'])) {
header('Location: http://site.com/notallowed');
die();
}