Pho implementation

This is a very, very simple job for a skilled php developer. I've a php file which generates an xml sitemap. It simply adds a line to the xml file, taking it from a table of my database. It works perfectly, and I do not want any changes on it. Simply, the file size of the generated xml sitemap, right now, is 24mb, with more then the 50.000 urls allowed by google webmaster tool. I want a developer to preserve the way the file works, but simply close the sitemap and generate a new one (by simply adding a 5 digit number) every time the sitempa reach 49.999 rows. No fpt access. But I'm online and I can upload and check if the file works correcty in real time. Very small budget, because the task should be very simple (maybe 5 minutes of work).