index.php

<?php
session_start();

//require all PHP files from a directory
$folderList = array(
    './config/',
    './app/core/',
    './lib/',
);
foreach ($folderList as $folder) {
    foreach (glob($folder . "*.php") as $filename) {
        require $filename;
    }
}

require './app/models/BaseModel.php';

spl_autoload_register(function ($className) {
    require './app/controllers/' . $className . '.php';
});

//Controller
if (isset($_REQUEST['controller']) && '' != $_REQUEST['controller']) {
    $controllerParam = strtolower($_REQUEST['controller']);
}
$controllerName = ucfirst(($controllerParam ?? '') . 'Controller');
require "./app/controllers/${controllerName}.php";

//Action
if (isset($_REQUEST['action']) && '' != $_REQUEST['action']) {
    $actionParam = strtolower($_REQUEST['action']);
}
$actionName = $actionParam ?? 'index';

//Run
$controllerObject = new $controllerName;
$controllerObject->$actionName();