Add the featured image to RSS feed(s)

add-featured-image-to-rss-feed.php

<?php
/**
 * Add Featured Image to RSS Feed
 *
 * @package           AddFeaturedImageToRssFeed
 * @author            Micah Wood
 * @copyright         Copyright 2023 by Micah Wood - All rights reserved.
 * @license           GPL2.0-or-later
 *
 * @wordpress-plugin
 * Plugin Name:       Add Featured Image to RSS Feed
 * Plugin URI:        https://gist.github.com/wpscholar/0ddf40fa3a72456e718f84e50ef90677
 * Description:       Adds featured images to all RSS feeds, if set.
 * Version:           1.0
 * Requires PHP:      7.4
 * Requires at least: 6.0
 * Author:            Micah Wood
 * Author URI:        https://wpscholar.com
 * Text Domain:       add-featured-image-to-rss-feed
 * Domain Path:       /languages
 * License:           GPL V2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 */

function addFeaturedImageToFeed( $content ) {
	if ( has_post_thumbnail( get_the_ID() ) ) {
		$content = '<div>' . get_the_post_thumbnail( get_the_ID(), 'large', [ 'style' => 'margin-bottom: 15px;' ] ) . '</div>' . $content;
	}

	return $content;
}

add_filter( 'the_excerpt_rss', 'addFeaturedImageToFeed' );
add_filter( 'the_content_feed', 'addFeaturedImageToFeed' );