I would simply look for a $_GET
variable and redirect the user if it's not correct.
<?php
$pass = $_GET['pass'];
if($pass != 'my-secret-password') {
header('Location: http://www.staggeringbeauty.com/');
}
?>
Now, if this page is located at say: http://example.com/secrets/files.php
You can now access it with: http://example.com/secrets/files.php?pass=my-secret-password
Keep in mind that this isn't the most efficient or secure way, but nonetheless it is a easy and fast way. (Also, I know my answer is outdated but someone else looking at this question may find it valuable)