If I understand correctly that you want to allow them to only be able to access your index/root document (www.mydomain.com/index.php etc.) and not be able to type in: www.mydomain.com/aboutus.php it is fairly simple using the HTTP referrer to make sure that the page they came from was the right one:
Important note (edit): The $_SERVER type variables are susceptible to forgery by the client from something like cURL or even telnet and can be the basis for CSRF type attacks, so this is by no means a secure implementation vs. something like session tokenization.
aboutus.php at the very top:
<?php
// Put the url they came from
$ref = $_SERVER['HTTP_REFERER'];
if($ref !== 'http://mydomain.com/index.php') {
die("Must come here from index");
// uncomment below to redirect them automatically
// header('location: index.php');
}
// Otherwise they came from index so show the page.
echo "Displaying about page:";
echo $content;
?>