Refactored Auto Featured Image.

auto_featured_image_refactored.php

<?php
class First_Image_To_Thumbnail{
	public function __construct(){
		add_action('transition_post_status', array($this, 'autoset_featured'), 10, 3 );
	}

	public function search_post_content_first_item( $tag, $post ){
		$dom = new DOMDocument();
		@$dom->loadHTML( $post->post_content );
		$search_dom = new DOMXPath($dom);
		$found_tag = $search_dom->query('//'.$tag.'');
		$is_found_tag = $found_tag->length;

		if ( !$is_found_tag ) {
			return FALSE;
		}
		return $found_tag;
	}
	public function item_has_class($found_tag,$class_to_match){
		$item_class = $found_tag->item(0)->getAttribute( 'class' );
		$do_classes_match = strpos( $item_class, $class_to_match );
		if($do_classes_match == FALSE ){
			return FALSE;}
		else{
			return TRUE;
		}
	}

	public function side_load_found_image_as_thumb($found_image, $post){
		$image_src = $found_image->item(0)->getAttribute( 'src' );
		$tmp = download_url( $image_src );
		$file_array = array(
			'name' => basename( $image_src ),
			'tmp_name' => $tmp
		);
		if ( is_wp_error( $tmp ) ) {
			@unlink( $file_array[ 'tmp_name' ] );
			return $tmp;
		}
		$added_attachment = media_handle_sideload($file_array, $post->ID);
		set_post_thumbnail( $post->ID, $added_attachment );
		return;
	}

	public function match_found_image_with_attached( $found_image, $post ){
		$attached_images = get_attached_media( 'image' );
		$imageclass = $found_image->item(0)->getAttribute( 'class' );
		foreach ( $attached_images as $attachment_id => $attachment ) {
			$current_attachment   = 'wp-image-' . $attachment_id;
			$first_attached_image = strpos( $imageclass, $current_attachment );
			if ( $first_attached_image !== FALSE ) {
				return $attachment_id;
			}
			return $first_attached_image;
		}
	}

	public function guess_attachment_based_on_class($found_image){
		$imageclass = $found_image->item(0)->getAttribute( 'class' );
		$guess_attach_id = explode( ' wp-image-',$imageclass  );
		$maybe_attachment = $guess_attach_id[1];
		$is_attachment_image = wp_attachment_is_image( $maybe_attachment );
		if( !$is_attachment_image ){
			return $is_attachment_image;
		}
		return $maybe_attachment;
	}

	public function should_autoset_run($new_status, $old_status, $post){
		if ( $new_status != 'publish' || wp_is_post_autosave( $post->ID ) || $post->post_type != 'post' || empty( $post->post_content || has_post_thumbnail( $post->ID ) )) {
			return FALSE;
		}

		$already_has_thumb = has_post_thumbnail( $post->ID );
		if($already_has_thumb){
			return FALSE;
		}
		return TRUE;
	}

	public function autoset_featured( $new_status, $old_status, $post ) {

		if(!$this->should_autoset_run($new_status, $old_status, $post)){
			return;
		}

		$found_image = $this->search_post_content_first_item('img', $post );

		if(false == $found_image){
			return;
		}

		if(!$this->item_has_class($found_image,'wp-image-')){
			$this->side_load_found_image_as_thumb($found_image, $post);
			return;
		}
		$attached_images = get_attached_media( 'image' );
		if ( $attached_images ) {
			$attachment_match = $this->match_found_image_with_attached( $found_image, $post );
			if($attachment_match !== FALSE ){
				set_post_thumbnail( $post->ID, $attachment_match );
				return;
			}
		}
		$maybe_attachment = $this->guess_attachment_based_on_class($found_image);
		if(!$maybe_attachment == false){
			set_post_thumbnail( $post->ID, $maybe_attachment );
		}
		return;
	}
}
$add_thumbnails = new First_Image_To_Thumbnail();