Filter in a URL to the pingback stack in WordPress

filter-in-pingback.php

<?php
function pingback_format_link_url($post_links, $post_id) {
	$url = get_post_meta($post_id, '_format_link_url', true);
	if (!empty($url) && !in_array($url, $post_links)) {
		$post_links[] = $url;
	}
	return $post_links;
}
add_filter('pre_ping_post_links', 'pingback_format_link_url', 10, 2);