Automatically Set Featured Image On A Post

wp default post featured image

<?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');
?>