sak.module

<?php
/**
 * @file
 * Convert given basic page node to json using api key.
 * @author : Sunny.
 */
/**
 * Implements hook_menu().
 */
function sak_menu() {
  $menu['node_json/%'] = array(
      'title' => 'Basic node to json',
      'page callback' => 'drupal_get_form',
      'page arguments' => array('sak_logic'),
      'access callback' => TRUE,
      'type' => MENU_CALLBACK,
  );
  return $menu;
}
/**
 * Convert given node to its json value.
 */
function sak_logic() {
  $arg = arg();
     // for URL node_json only
  if ($arg[0] == 'node_json' && $arg[1] != NULL && is_numeric($arg[1])) {
    if (variable_get('siteapikey') != '' and variable_get('siteapikey') != NULL ) {
     if ($node = node_load ($arg[1]) ) {
        // Should be 'Page' content type only and should be published.
        if ( $node->type == 'page' && ($node->status ==1) ) {
          // Display JSON data of given node
          drupal_add_http_header('Content-Type', 'application/json');
          if (isset($node)) {
            echo drupal_json_encode($node);
            exit();
          }
        }   
        else
        {
          drupal_set_message('Access Denied' , 'error');
        }
      }
    }
  }
}

/**
 * Implements hook_form_alter().
 */
function sak_form_alter(&$form, &$form_state, $form_id) {
  if ( $form_id == 'system_site_information_settings' ) {
       // Textbox for API secret key
    $form['site_information']['siteapikey'] = array(
      '#type' => 'textfield',
      '#title' => t('Site API Key'),
      '#default_value' => variable_get('siteapikey'),
      '#description' => t('Enter your API key here.'),
      '#required' => TRUE,
    );

    // If API Key value already set then Update Configuration.
    if ( variable_get('siteapikey') != NULL ) {
      $form['actions']['submit']['#value'] = t('Update Configuration');
    }

    // adding custom form submit in form.
    $form['#submit'][] = 'sak_settings_form_submit';

  }
  return system_settings_form($form);
}
/**
 * Form submit for sak_settings_form_submit.
 */
function sak_settings_form_submit(&$form, &$form_state) {
  // Set Drupal set meeage when field is saved.
  if ($form_state['values']['siteapikey'] != NULL || $form_state['values']['siteapikey'] != ''){
      $message = t('Site API Key has been saved');
      drupal_set_message($message);
   }
}