customize-feed-content.php

<?php
/*
Plugin Name: Customize Feed Content
Description:  I'm looking for a theme-independent plug-in that will let me customize my blog post excerpts for my RSS feed. I would like to be able to show the featured image and customize the length of the preview, as well as add a link underneath the excerpt for a viewer to click on to see the original article.
Version: 1.0
Author: artlung
*/
class CustomizeFeedContent {

	/**
	 * @param $content
	 *
	 * @return string
	 */
	public static function addFeaturedImage( $content ) {
		$featured_image_url = get_the_post_thumbnail_url(get_the_ID(), '200');
		$featured_image = '';
		if ($featured_image_url) {
			$featured_image = sprintf('<figure><img src="%s" alt="Featured Image" /></figure>', $featured_image_url);
		}
		return $featured_image . $content;
	}

	/**
	 * @param string $content
	 *
	 * @return string
	 */
	public static function addReadMoreLink( string $content ) {
		$permalink = get_permalink();
		$read_more = sprintf('<p><a href="%s">Read all of "%s" on "%s"</a></p>', $permalink, get_the_title(), get_bloginfo('name') );
		return $content . $read_more;
	}
}
/**
 * Adds any featured image to the full feed content.
 *
 * @param string $content The current content of the full feed.
 * @return string The modified content of the full feed.
 */
function customize_full_feed_content($content): string {
	$content = CustomizeFeedContent::addFeaturedImage($content);
	return $content;
}
add_filter('the_content_feed', 'customize_full_feed_content' );

/**
 * Adds any featured image to the excerpt feed content.
 * Also adds a "Read More" link to the full article.
 * @param $content
 * @return string
 */
function customize_exerpt_feed_content($content): string {
	$content = CustomizeFeedContent::addFeaturedImage($content);
	$content = CustomizeFeedContent::addReadMoreLink($content);
	return $content;
}
add_filter('the_excerpt_rss', 'customize_exerpt_feed_content' );