A custom plugin to output the years sitemap along the Yoast sitemaps, for Andy Myers Lodge

aml-yoast-years-custom-sitemap.php

<?php
/**
 * Plugin Name: AML Years XML Sitemap
 * Plugin URI: https://gist.github.com/alexmoise/66921b63fe6efe541eeb094118d1720a
 * GitHub Plugin URI: https://gist.github.com/alexmoise/66921b63fe6efe541eeb094118d1720a
 * Description: A custom plugin to output the years sitemap along the Yoast sitemaps, for Andy Myers Lodge
 * Version: 1.0.2
 * Author: Alex Moise
 * Author URI: https://moise.pro
 */

// Add the sitemap to Yoast sitemaps index
add_filter( 'wpseo_sitemap_index', 'AML_sitemap_custom_items' );
function AML_sitemap_custom_items() {
 $sitemap_custom_items = '
  <sitemap>
   <loc>'.get_home_url().'/years-sitemap.xml</loc>
   <lastmod>2017-05-22T23:12:27+00:00</lastmod>
  </sitemap>
 ';
 return $sitemap_custom_items;
}

// === Now about the sitemap ===
// On publish of post, update sitemap
add_action("publish_post", "AML_years_sitemap_gen");
// Update of page
add_action("publish_page", "AML_years_sitemap_gen");
// If revised post/page
add_action("save_post", "AML_years_sitemap_gen");
// On activation, create sitemap
register_activation_hook( __FILE__, 'AML_years_sitemap_gen');

function AML_years_sitemap_gen() {
	global $Gallery;
	$homepath = get_home_path();
	// Create XML
	$sitemap = '<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//'.parse_url(get_home_url(), PHP_URL_HOST).parse_url(get_home_url(), PHP_URL_PATH).'/main-sitemap.xsl"?>';
	$sitemap .= "\n".'<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">'."\n";
		foreach($Gallery->archives_years() as $year) {
			$sitemap .= "\t".'<url>'."\n".
				"\t\t".'<loc>'. $year['archive_link'] .'</loc>'.
				"\n\t\t".'<changefreq>monthly</changefreq>'.
				"\n\t".'</url>'."\n";
		}
	$sitemap .= '</urlset>';
	$fp = fopen($homepath . "years-sitemap.xml", 'w');
	fwrite($fp, $sitemap);
	fclose($fp);
}

?>