Drupal 7 — Basic Ajax request (Ajax framework)

demo.module

<?php

/**
 * @file
 * Demo module, Basic Ajax request (Ajax framework).
 */

/**
 * Implements hook_menu().
 */
function demo_menu() {
  return array(
    'demo/page' => array(
      'page callback' => 'demo_page_callback',
      'access callback' => TRUE,
      'type' => MENU_CALLBACK,
    ),
    'demo/ajax' => array(
      'page callback' => 'demo_ajax_callback',
      'access callback' => TRUE,
      'type' => MENU_CALLBACK,
    ),
  );
}

/**
 * Demo page callback.
 */
function demo_page_callback() {
  drupal_add_library('system', 'drupal.ajax');

  $output = '<p>This is an ajax demo link: ';
  $output .= l('Click me ! ', 'demo/ajax/nojs/', array('attributes' => array('class' => array('use-ajax'))));
  $output .= '</p>';

  return $output;
}

/**
 * Demo Ajax callback.
 */
function demo_ajax_callback($type = 'ajax') {
  if ($type == 'ajax') {
    $output = 'Hello ! ';
    $commands = array();
    $commands[] = ajax_command_append('#block-system-main .content', $output);
    $response = array('#type' => 'ajax', '#commands' => $commands);
    ajax_deliver($response);
  }
  else {
    $output = 'Hello ! (no js) ';
    return $output;
  }
}