better_guestbook/api/auth/isauth.php

<?php
// Set internal to false so that the answer goes to the client
if (!isset($internal)) {
    $internal = false;
}

// Load Main Library
require "../api.php";
$api = new gbAPI(true);

$api->answer(200, [
    "login" => true,
    "username" => $api->username
]);