<?php
error_reporting(E_ALL | E_STRICT);

function menu_get_item() {
  return array(
    'callbacks' => array(
      'menu_load' => array(
        'callback' => 'node_load',
        'arguments' => array(array('arg', 1)),
        'store' => array('node'),
        'transition' => array(
          'menu_not_found' => FALSE,
          'menu_access' => 'default',
        ),
      ),
      'menu_access' => array(
        'callback' => 'node_access',
        'arguments' => array(array('node')),
        'transition' => array(
          'menu_page' => 'default',
          'menu_access_denied' => FALSE,
        ),
      ),
      'menu_page' => array(
        'callback' => 'node_page_view',
        'arguments' => array(array('node', 'nid')),
        'store' => array('regions', 'content'),
      ),
      'menu_not_found' => array(
        'callback' => 'drupal_not_found',
      ),
      'menu_access_denied' => array(
        'callback' => 'drupal_not_found',
      ),
    ),
  );
}

function arg($index, $path) {
  return explode('/', $path);
}

function node_load() {
  $node = new stdClass();
  $node->nid = 123;
  return $node;
}

function node_access($node) {
  return TRUE;
}

function node_page_view($nid) {
  return 'node content';
}

$path = $_GET['q'] = 'node/4';
$user = 'user object';

if ($router_item = menu_get_item($path)) {
  $callbacks = /* unserialize */ ($router_item['callbacks']);
  $callback = reset($callbacks);
  $context = array(
    'arg' => arg(NULL, $_GET['q']),
    'user' => $user,
  );
  while ($callback) {
    $function = $callback['callback'];
    if ($callback['arguments']) {
      foreach ($callback['arguments'] as &$arguments) {
        if (!isset($arguments)) {
          $arguments = $context;
        }
        if (is_array($arguments)) {
          $value = $context;
          foreach ($arguments as $index) {
            $value = is_array($value) ? $value[$index] : $value->$index;
          }
          $arguments = $value;
        }
      }
      $result = call_user_func_array($function, $callback['arguments']);
    }
    else {
      if ($function == 'exit') {
        exit();
      }
      $result = $function();
    }
    if (isset($callback['store'])) {
      $store = &$context;
      foreach ($callback['store'] as $index) {
        $store = &$store[$index];
      }
      $store = $result;
      unset($store);
    }
    if (isset($callback['transition'])) {
      unset($default);
      foreach ($callback['transition'] as $step => $condition) {
        if ($result == $condition) {
          $callback = $callbacks[$step];
          continue 2;
        }
        if ($condition == 'default') {
          $default = $step;
        }
      }
      if (isset($default)) {
        $callback = $callbacks[$step];
        continue;
      }
    }
    return $result;
  }
}