index.php

<?php

require_once "config.php";

function __autoload($file)
{
    if (file_exists('controller/' . $file . '.php')) {
        require_once 'controller/' . $file . '.php';
    } elseif (file_exists('model/' . $file . '.php')) {
        require_once 'model/' . $file . '.php';
    } elseif (file_exists('API/' . $file . '.php')) {
        require_once 'API/' . $file . '.php';
    } elseif (file_exists('model/BookRepository/' . $file . '.php')) {
        require_once 'model/BookRepository/' . $file . '.php';
    } elseif (file_exists('model/AuthorRepository/' . $file . '.php')) {
        require_once 'model/AuthorRepository/' . $file . '.php';
    } elseif (file_exists('model/CategoryRepository/' . $file . '.php')) {
        require_once 'model/CategoryRepository/' . $file . '.php';
    }
}

if (isset($_GET['book'])) {
    $init = new BookById();
} elseif (isset($_GET['authors']) && (!empty($_GET['authors'])) ||
            (isset($_GET['categories']) && (!empty($_GET['categories'])))) {
    $init = new Filter();
} else {
    $init = new Index();
}

echo $init->execute();