Drupal: Add node extra field

gistfile1.php

<?php


/**
 * Implements hook_field_extra_fields().
 */
function mymodule_field_extra_fields() {
  // $extra['entity_type']['bundle']
  $extra['node']['my_content_type'] = array(
    'display' => array(
      'my_extra_field' => array(
        'label' => t("My Extra field"),
        'description' => t("This is my extra field."),
        'weight' => 10, // Default weight
      ),
    )
  );
  return $extra;
}


/**
 * Implements hook_node_view().
 */
function mymodule_node_view($node, $view_mode, $langcode) {
  // Get extra field status
  $extra_fields = field_extra_fields_get_display('node', $node->type, $view_mode);

  if ($node->type == 'my_content_type') {
    if (!empty($extra_fields['my_extra_field']['visible'])) {
      // Add extra field render array to node content
      $node->content['my_extra_field'] = array(
        '#markup' => "My extra field content."
      );
    }
  }
}