Not to long a go we explained how to generate a sitemap.xml with PHP.
Well, there was something missing and I think really important, to ping Google about the changes in the sitemap.
Simplest way of doing this:
file_get_contents('http://www.google.com/webmasters/sitemaps/ping?sitemap=http://yoursite.com/sitemap.xml');
Remember to register your site at Google webmaster tools, since if you don’t will not work.