Add thumbnail to WordPress RSS with another tag

I solved it finally :) I changed the function that I posted before. The new function is this:

add_action('rss2_item', function(){  global $post;  $output = '';  $thumbnail_ID = get_post_thumbnail_id( $post->ID );  $thumbnail = wp_get_attachment_image_src($thumbnail_ID, 'thumbnail');  $output .= '<post-thumbnail>';    $output .= '<url>'. $thumbnail[0] .'</url>';    $output .= '<width>'. $thumbnail[1] .'</width>';    $output .= '<height>'. $thumbnail[2] .'</height>';    $output .= '</post-thumbnail>';  echo $output;});

This gives the image link in new tag <post-thumbnail> like I wanted.

There are 2 functions in wordpress that can help you in a very easy way, try this:

(int) $id = get_post_thumbnail_id($post->ID);$url = wp_get_attachment_url( $id ); 

Edit:

To modify the feed you can check this documentation

One easy option is modifying directly the feed template, an example:

in file wp-includes/feed-rss2.php (note you may modify another feed template as "feed-rss.php", "feed-atom.php") you can add the new tag:

echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>'; ?><rss version="2.0"    xmlns:content="http://purl.org/rss/1.0/modules/content/"    xmlns:wfw="http://wellformedweb.org/CommentAPI/"    xmlns:dc="http://purl.org/dc/elements/1.1/"    xmlns:atom="http://www.w3.org/2005/Atom"    xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"    xmlns:slash="http://purl.org/rss/1.0/modules/slash/"    <?php do_action('rss2_ns'); ?>><channel>    <title><?php bloginfo_rss('name'); wp_title_rss(); ?></title>    <atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />    <link><?php bloginfo_rss('url') ?></link>    <description><?php bloginfo_rss("description") ?></description>    <lastBuildDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_lastpostmodified('GMT'), false); ?></lastBuildDate>    <language><?php bloginfo_rss( 'language' ); ?></language>    <sy:updatePeriod><?php echo apply_filters( 'rss_update_period', 'hourly' ); ?></sy:updatePeriod>    <sy:updateFrequency><?php echo apply_filters( 'rss_update_frequency', '1' ); ?></sy:updateFrequency>    <?php do_action('rss2_head'); ?>    <?php while( have_posts()) : the_post(); ?>    <item>        <title><?php the_title_rss() ?></title>        <link><?php the_permalink_rss() ?></link>        <comments><?php comments_link_feed(); ?></comments>        <pubDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_post_time('Y-m-d H:i:s', true), false); ?></pubDate>        <dc:creator><?php the_author() ?></dc:creator>        <?php the_category_rss('rss2') ?>        <?php             //start modified code to add tag with post thumb url            (int) $id = get_post_thumbnail_id($post->ID);            $thumb_url = wp_get_attachment_url( $id );         ?>        <thumb><?php echo $thumb_url;?></thumb>            <?php //end modified code to add tag with post thumb url ?>        <guid isPermaLink="false"><?php the_guid(); ?></guid><?php if (get_option('rss_use_excerpt')) : ?>        <description><![CDATA[<?php the_excerpt_rss() ?>]]></description><?php else : ?>        <description><![CDATA[<?php the_excerpt_rss() ?>]]></description>    <?php if ( strlen( $post->post_content ) > 0 ) : ?>        <content:encoded><![CDATA[<?php the_content_feed('rss2') ?>]]></content:encoded>    <?php else : ?>        <content:encoded><![CDATA[<?php the_excerpt_rss() ?>]]></content:encoded>    <?php endif; ?><?php endif; ?>        <wfw:commentRss><?php echo esc_url( get_post_comments_feed_link(null, 'rss2') ); ?></wfw:commentRss>        <slash:comments><?php echo get_comments_number(); ?></slash:comments><?php rss_enclosure(); ?>    <?php do_action('rss2_item'); ?>    </item>    <?php endwhile; ?></channel></rss>

There is a plugin to do this for WordPress that adds the image as an enclosure.

https://github.com/kasparsd/feed-image-enclosure/blob/master/feed-image-enclosure.php