Basically it's a thing that can be run from cron, and generates sitemaps which then overwrites what is on the sitemap.xml
I'm thinking also, can google access a .php file such as sitemap.php and this would generate a sitemap. And because google bot visits this sitemap, this php file would immediately query the site about it's links, thus making this process efficient.
Is any of these ways feasible? And how do I go about getting making a sitemap generator (that is in a php script), is there any freely downloadable ones out there?