“Proper” separation/difference between index.php and front controller

Question

For a PHP MVC application, what is the difference of the job of the index.php file and front-controller? Is the front-controller in the index.php,

Answer 1

Actually, index.php should not contain any meaningful code at all, since it would be only part of your site, that is located inside DOCUMENT_ROOT of webserver. It's content should actually look something like:

It should only include a file outside DOCUMENT_ROOT. And that's all.

This way, if something goes horribly wrong (like, php extension fails after server update) and visitors are exposed to raw php code, it will not reveal any sensitive details.

The point of Front Controller is handle all user input, turn it into a consumable form and, based on it, dispatch a command (usually in a form of method call on an object). In languages like Java, where everything must be contained in a class, a front controller would be a class. But in php you do not have this restriction.

Instead the front controller will end up being part of your bootstrap stage of the application:

// --- snip --- 
// the autoloader has been initialized already a bit earlier

$router = new Router;
$router->loadConfig($configuration);

$request = new Request;
$request->setUri($GET['url']); 
// could also be $_SERVER['PATH_INFO'] or other
// depends on how url rewrite is set up

$router->route($request);
// the request instance is populated with data from first matching route

$class = $request->getParameter('resource');
$command = $request->getMethod() . $request->getParameter('action');

if (class_exists($class)) {
    $instance = new $class;
    $instance->{$command}($request);
    // you dispatch to the proper class's method 
}

// --- snip --- 
// then there will be some other code, unrelated to front controller

Also, you should keep in mind that concept of front controller is neither made-for nor demanded-by application that attempt to implement MVC or MVC-inspired architecture.