Adds custom XML tags to the default WP RSS feed output.

functions.php

<?php

/*
  Adds custom tags to the default WP RSS feed output
  Reference: https://codex.wordpress.org/Plugin_API/Action_Reference/rss2_item
*/

function rss_add_custom_tags() {
    global $post;

    // add post featured image to the feed item if present
    if(has_post_thumbnail($post->ID)):
        $thumbnail = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'full');
        echo"\t<image>{$thumbnail[0]}</image>\n";
    endif;

    // add post categories if present
    $cats = wp_get_post_categories($post->ID, array('fields' => 'all'));
    foreach ($cats as $cat) echo"\t<categoryLink>" . get_category_link($cat->term_id) . "::" . $cat->name . "</categoryLink>\n";

    // add post tags if present
    $tags = wp_get_post_tags($post->ID);
    foreach ($tags as $tag) echo"\t<tagLink>" . get_tag_link($tag->term_id). "::" . $tag->name . "</tagLink>\n";
}
add_action('rss2_item', 'rss_add_custom_tags');
add_action('rss_item', 'rss_add_custom_tags');