WordPress RSS feed post thumbnail for MailChimp

feed-thumbnail.php

<?php

/**
 * This file can be dropped in wp-content/mu-plugins/ to be activated.
 * It will add post thumbnails to the RSS feed.
 * The image can then be used in MailChimp templates with the *|RSSITEM:IMAGE|* merge tag.
 * This implementation is based on useful indications by [vick](https://stackoverflow.com/a/48549541)
 */

// add xml namespace for media element
add_action('rss2_ns', function () {
    echo 'xmlns:media="http://search.yahoo.com/mrss/"';
});

// insert media element inside rss items when post has a thumbnail
add_action('rss2_item', function () {
    global $post;
    if (!$post || !has_post_thumbnail($post)) {
        return;
    }
    $pid = get_post_thumbnail_id($post->ID);
    if (!$pid) {
        return;
    }
    $src = wp_get_attachment_image_src($pid, 'large');
    printf(
        '<media:content medium="image" url="%s" width="%s" height="%s" />',
        wp_get_attachment_image_url($pid, 'large'),
        $src[1],
        $src[2]
    );
});