To resubmit your Sitemap using an HTTP request:
Issue your request to the following URL:
www.google.com/webmasters/tools/ping?sitemap=sitemap_url
www.google.com/webmasters/tools/ping?sitemap=http://www.example.com/sitemap.gz
www.google.com/webmasters/tools/ping?sitemap=http%3A%2F%2Fwww.yoursite.com%2Fsitemap.gz
Code
I got the code from here
http://nadeausoftware.com/articles/2007/06/php_tip_how_get_web_page_using_curl#curlinit
The quick code
$result = get_web_page('www.google.com/webmasters/tools/ping?sitemap='.urldecode('http://site.com/sitemap.xml'));
if ( $result['errno'] != 0 )
echo "error , bad url";
if ( $result['http_code'] != 200 )
echo "error , no servivice";
$page = $result['content'];
echo $page;
?>
output
"Sitemap Notification Received
Your Sitemap has been successfully added to our list of Sitemaps to crawl. If this is the first time you are notifying Google about this Sitemap, please add it via http://www.google.com/webmasters/tools/ so you can track its status. Please note that we do not add all submitted URLs to our index, and we cannot make any predictions or guarantees about when or if they will appear."
After this, put a cron tab to the link , Note that please dont go less than a hour request to google. otherwise they will be angry.