On my index.php page of my main root a user gets a betacode. Which is $betacode. And then on another page, i.e. /private/index.php, how would I get it to show that beta code. Like something like include '../index.php'; $_GET['betacode']) how would I so, because right now it's not working.

If it's in the same request and your $betacode is declared outside of a function the variable will be available in any page you include after that. If you are redirecting them to the second page you can either pass it through the URL or use the a session variable.