An example of an MVC controller

Request example

Put something like this in your index.php:

<?php

// Holds data like $baseUrl etc.
include 'config.php';

$requestUrl = 'http://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];
$requestString = substr($requestUrl, strlen($baseUrl));

$urlParams = explode('/', $requestString);

// TODO: Consider security (see comments)
$controllerName = ucfirst(array_shift($urlParams)).'Controller';
$actionName = strtolower(array_shift($urlParams)).'Action';

// Here you should probably gather the rest as params

// Call the action
$controller = new $controllerName;
$controller->$actionName();

Really basic, but you get the idea... (I also didn't take care of loading the controller class, but I guess that can be done either via autoloading or you know how to do it.)

Simple controller example (controllers/login.php):

<?php    

class LoginController
{
    function loginAction()
    {
        $username = $this->request->get('username');
        $password = $this->request->get('password');

        $this->loadModel('users');
        if ($this->users->validate($username, $password))
        {
            $userData = $this->users->fetch($username);
            AuthStorage::save($username, $userData);
            $this->redirect('secret_area');
        }
        else
        {
            $this->view->message = 'Invalid login';
            $this->view->render('error');
        }
    }

    function logoutAction()
    {
        if (AuthStorage::logged())
        {
            AuthStorage::remove();
            $this->redirect('index');
        }
        else
        {
            $this->view->message = 'You are not logged in.';
            $this->view->render('error');
        }
    }
}

As you see, the controller takes care of the "flow" of the application - the so-called application logic. It does not take care about data storage and presentation. It rather gathers all the necessary data (depending on the current request) and assigns it to the view...

Note that this would not work with any framework I know, but I'm sure you know what the functions are supposed to do.

Imagine three screens in a UI, a screen where a user enters some search criteria, a screen where a list of summaries of matching records is displayed and a screen where, once a record is selected it is displayed for editing. There will be some logic relating to the initial search on the lines of

if search criteria are matched by no records
    redisplay criteria screen, with message saying "none found"
else if search criteria are matched by exactly one record
    display edit screen with chosen record
else (we have lots of records)
    display list screen with matching records

Where should that logic go? Not in the view or model surely? Hence this is the job of the controller. The controller would also be responsible for taking the criteria and invoking the Model method for the search.