WordPress: Add Featured Image to RSS feed, works with Active campaign feeds

wp_rss_add_thumbnail_to_rss_feed_for_active_campaign.php

<?php
/**
 * WordPress
 * Add Featured Image to RSS feed
 * works with Active campaign feeds
**/
// haha.nl - herbert hoekstra - 20201210

function hahadev_rss_add_thumbnail() {
        global $post;

        $output = '';
        if ( has_post_thumbnail( $post->ID ) ) {
            $thumbnail_ID = get_post_thumbnail_id( $post->ID );
            $thumbnail = wp_get_attachment_image_src( $thumbnail_ID, 'thumbnail' );

            $output .= '<enclosure medium="image" type="image/jpeg"';
            $output .= ' url="'. urlencode($thumbnail[0]) .'"';
            $output .= ' width="'. $thumbnail[1] .'"';
            $output .= ' height="'. $thumbnail[2] .'"';
            $output .= ' />';
        }
        echo $output;
    }
add_action( 'rss2_item', 'hahadev_rss_add_thumbnail' );