How to Set the First Post Image as a Featured Image in WordPress

You installed a new theme and you have a lot of posts without a featured image and want to change that? This script will take care of your featured image problem, all you need to do is copy the code into your theme’s functions.php file. Be aware, it will not add a new image if you don’t have one already in the post and it will not change your current featured images.

function auto_featured_image() {
    global $post;
    if (!has_post_thumbnail($post->ID)) {
        $attached_image = get_children( "post_parent=$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);
// 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');