Wordpress: почему не работает пользовательский фид?

add_action( 'init', function(){
	add_feed( 'smartprice', 'smartprice_markup' );
});
function smartprice_markup() {
	do_action( 'my_before_feed' );
	header( 'Content-Type: ' . feed_content_type( 'rss' ) . '; charset=' . get_option( 'blog_charset' ), true );
	status_header( 200 );
	echo '<?xml version="1.0" encoding="UTF-8"?>';
	?>
	<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:sy="http://purl.org/rss/1.0/modules/syndication/"
		 xmlns:slash="http://purl.org/rss/1.0/modules/slash/">
		<channel>
			<?php 
			
			...

			?>
		</channel>
	</rss>
	<?php
	exit;
}