The abstract feed class

feed.php

<?php

namespace Postmedia\Web;

abstract class Feed {

	/**
	 * Constructor
	 * @return object instance
	 */
	public function __construct() {
		add_action( 'pre_get_posts', array( $this, 'remove_sponsored_posts_from_feed' ), 1 );
	}

	/**
	 * Remove sponsored posts from feeds.
	 */
	public function remove_sponsored_posts_from_feed( $query ) {
		if ( ! is_feed() ) {
			return null;
		}

		if ( $query->is_main_query() ) {
			$query->set( 'post_type', 'post' );
			$query->set(
				'meta_query',  array(
					'relation' => 'AND',
					array(
						'key' => 'advertorial_meta_box',
						'compare' => 'NOT EXISTS',
					),
					array(
						'key' => 'pn_adv_sponsor_name',
						'compare' => 'NOT EXISTS',
					),
				)
			);
		}
	}

	/**
	 * This Feed class allows a developer to use this default Feed header.
	 * When inheriting a developer can override the get_custom_feed_base_header() method to
	 * provide new content types, and namespaces.
	 * @return string
	 */
	public function get_custom_feed_base_header() {
		header( 'Content-Type: ' . feed_content_type( 'rss-http' ) . '; charset=' . get_option( 'blog_charset' ), true );

		$feed_header = '<?xml version="1.0" encoding="' . get_option( 'blog_charset' ) . '"?' . '>' . "\n\n";
		$feed_header .= '<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/"
		>' . "\n\n";

		return $feed_header;
	}
}