Add a class to the first image field of all blog articles on a Drupal 7 site.

updateBodyImageClass.php

<?php
/**
 * Created by PhpStorm.
 * User: chris
 * Date: 3/25/15
 * Time: 11:44 AM
 */

# Bootstrap start
define('DRUPAL_ROOT', '.');
//$_SERVER['REMOTE_ADDR'] = "localhost"; // Necessary if running from command line
require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
# Bootstrap end

$query = new EntityFieldQuery();
$result = $query->entityCondition('entity_type', 'node')
                ->entityCondition('bundle', 'blog')
                ->execute();
$nodeIds = array_keys($result['node']);

foreach ($nodeIds as $node_id) {
  $node_wrapper = entity_metadata_wrapper('node', $node_id);

  $body = $node_wrapper->body->value->raw();

  $new_body = add_class_to_first_img_tag ($body);

  $node_wrapper->body->value = $new_body;

  $node = $node_wrapper->value();
  field_attach_update('node', $node);

  //$node_wrapper->save();
  print $node_id . '<br/>';
}

function add_class_to_first_img_tag ($string) {
  if(strpos($string, '<img')){
    $split_point =  strpos($string, '<img') + 4;

    $first_half = substr($string, 0, $split_point);
    $second_half = substr($string, $split_point);

    $new_string = $first_half . ' class="legacy" ' . $second_half;
    return $new_string;
  } else {
    return $string;
  }
}