WordPress Plugin: Automatically Set the Featured Image

cdm-automatically-set-featured-image.php

<?php
/**
 * Plugin Name: Automatically Set the Featured Image
 * Plugin URI: https://gist.github.com/chrisdavidmiles/31fd0b6dfb4716ee9bfa34ca68b6cb57
 * Description: When a post is saved or updated, if a featured image is not set, the featured image will be set to the first image in the post. 
 * Author: Jonathan Dingman
 * Author URI: https://web.archive.org/web/20170926163348/http://wpforce.com/automatically-set-the-featured-image-in-wordpress/
 * Version: 0.1
 */

if ( ! defined( 'ABSPATH' ) )  die; 

// Set Featured Image - Function
function cdm_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);
          }
        }
    }
}

// Set Featured Image - Automation
add_action('the_post', 'cdm_autoset_featured');
add_action('save_post', 'cdm_autoset_featured');
add_action('draft_to_publish', 'cdm_autoset_featured');
add_action('new_to_publish', 'cdm_autoset_featured');
add_action('pending_to_publish', 'cdm_autoset_featured');
add_action('future_to_publish', 'cdm_autoset_featured');

?>