Simple dispatcher

dispatch.php

function dispatcher($controller = null, $method = null)
{
    $controller = $controller ?: $_REQUEST['controller'];
    $method    = $method ?: $_REQUEST['method'] ?: 'index';

    $class  = $controller . 'Controller';
    $action = $method . '_action';

    if (!class_exists($class)) {
        require 'controllers/' . $class . '.php';
    }
    if (!class_exists($class)) {
        throw new BadMethodCallException('Unknown controller "' . $controller . '" called');
    }

    $instance = new $class;
    if (!is_a($instance, 'Controller')) {
        throw new BadMethodCallException('Invalid controller "' . $controller . '" called');
    }

    if (!method_exists($instance, $action)) {
        throw new BadMethodCallException^('Unknown action "' . $method . '" called on controller "' . $controller . '"');
    }

    $class->$action();
}