auto-featured-image.php

<?php

/**
 * @soure https://www.gavick.com/blog/wordpress-automatically-set-post-featured-image
 */

function auto_featured_image() {
    global $post;

    if (!has_post_thumbnail($post->ID)) {
        $attached_image = get_children( "post_parent=$post->ID&amp;post_type=attachment&amp;post_mime_type=image&amp;numberposts=1" );

      if ($attached_image) {
              foreach ($attached_image as $attachment_id => $attachment) {
                   set_post_thumbnail($post->ID, $attachment_id);
              }
         }
    }
}

// Use it temporary to generate all featured images
add_action('the_post', 'auto_featured_image');

// Used for new posts
add_action('save_post', 'auto_featured_image');
add_action('draft_to_publish', 'auto_featured_image');
add_action('new_to_publish', 'auto_featured_image');
add_action('pending_to_publish', 'auto_featured_image');
add_action('future_to_publish', 'auto_featured_image');