Php function to clear cache for regions
You are looking for cache_clear_all. You want to set first argument $cid to NULL, as you want expire all your objects. Now you only need to figure second argument, $bin. For sure you have to clear cache_page bin. If it's not enough, look inside your database for cache_% tables (where % means any string, per SQL standard) to know what bins are available to you.

Last but not least, dynamic regions may be huge performance hit. Think twice if that's what you really need.