How to Automatically Set the First Image of a WordPress Post as the Featured Image

This snippet automatically sets the featured image by fetching the first image of the WordPress post.
*Note – if you choose a featured image, that will be displayed instead.

Instructions:
Add this to the functions.php file.

<?php
   function autoset_featured() {
      global $post;
      $already_has_thumb = has_post_thumbnail($post->ID);
      if (!$already_has_thumb)  {
         $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
         if ($attached_image) {
            foreach ($attached_image as $attachment_id => $attachment) {
               set_post_thumbnail($post->ID, $attachment_id);
            }
         }
      }
   }
   add_action('the_post', 'autoset_featured');
   add_action('save_post', 'autoset_featured');
   add_action('draft_to_publish', 'autoset_featured');
   add_action('new_to_publish', 'autoset_featured');
   add_action('pending_to_publish', 'autoset_featured');
   add_action('future_to_publish', 'autoset_featured');
?>

Related links:
http://wpforce.com/automatically-set-the-featured-image-in-wordpress/