<?php

define("SITEMAP", "/server_app/webroot/publish/web/sitemap.xml");
define("LAST_MODIFIED",
        (!file_exists("sitemap.xml") || filemtime(SITEMAP) >= filemtime("sitemap.xml"))
                ? filemtime(SITEMAP)
                : filemtime("sitemap.xml"));

$sitemap1 = new DOMDocument();

$sitemap1->load(SITEMAP);

if (file_exists("sitemap.xml")) {

  $sitemap2 = new DOMDocument();

  $sitemap2->load("sitemap.xml");

  $sitemapUrlset = $sitemap1->getElementsByTagName("urlset")->item(0);

  $urls = $sitemap2->getElementsByTagName("url");

  if (!is_null($urls) && $urls !== false) {

    foreach ($urls as $url) {

      $newUrl = $sitemap1->importNode($url, true);

      $sitemapUrlset->appendChild($newUrl);
    }
  }
}

header("content-type: application/xml; charset=UTF-8");
header("last-modified: " . date("D, d M Y H:i:s", LAST_MODIFIED) . " GMT");

echo $sitemap1->saveXML();