Creating multiple fields and field instances in an update hook

multiple_files_update_hook.php

// Define new fields
$new_fields = array(
  'field_NAME_OF_FIELD' => array(
    'entity_type' => 'node',
    'bundle' => 'page',
    'field_definition' => array(
      'type' => 'text',
      // Other field details here. Exact keys depend on the type of field being added.
      // Best thing to do is create one on your local exactly how you want it (through
      // the UI), and then running field_read_field() and copying data below.
    ),
    'instance_definition' => array(
      'label' => 'Field Label',
      // Here too, there are many keys. Exact options depend on the field.
      // You should do the same as above (create through the UI on your local)
      // then call field_read_instance() to get the details and copy them in
      // here.
      // NOTE: Make sure display is hidden if you don't want it rendered. 
    ),
  ),
  'field_NAME_OF_FIELD' => array(
    'entity_type' => 'node',
    'bundle' => 'page',
    'field_definition' => array(
      'type' => 'text',
      // Other field details here. Exact keys depend on the type of field being added.
      // Best thing to do is create one on your local exactly how you want it (through
      // the UI), and then running field_read_field() and copying data below.
    ),
    'instance_definition' => array(
      'label' => 'Field Label',
      // Here too, there are many keys. Exact options depend on the field.
      // You should do the same as above (create through the UI on your local)
      // then call field_read_instance() to get the details and copy them in
      // here.
      // NOTE: Make sure display is hidden if you don't want it rendered. 
    ),
  ),
);

foreach ($new_fields as $field_name => $field_settings) {
  $entity_type = $field_settings['entity_type'];
  $bundle = $field_settings['bundle'];
  $field_settings['field_definition']['field_name'] = $field_name;
  $field_settings['instance_definition']['field_name'] = $field_name;
  // Add the new field if it doesn't already exist
  if (!$field = field_read_field($field_name)) {
    try {
      // We save of the result of field_create_field() for use later. See below.
      $field = field_create_field($field_settings['field_definition']);
      print('Created the "' . $field_name . '" field.');
    }
    catch (FieldException $e) {
      throw new DrupalUpdateException('Error creating field "field_form_name" with message: ' . $e->getMessage());
    }
  }

  // Don't bother if the field already exists.
  if (!field_read_instance($entity_type, $field_name, $bundle)) {
    // Apply bundle-specific settings.
    $field_settings['instance_definition']['entity_type'] = $entity_type;
    $field_settings['instance_definition']['bundle'] = $bundle;

    try {
      field_create_instance($field_settings['instance_definition']);
      print('Created a "' . $field_name . '" field instance for the "' . $bundle . '" ' . $entity_type . ' bundle.');
    }
    catch (FieldException $e) {
      throw new DrupalUpdateException('Error creating field instance for the "' . $bundle . '" bundle: ' . $e->getMessage());
    }
  }
}