Page caching

Is there any way that I can tell my PHP not to be cached? I've got a page that produces a map, and there are zoom buttons on it. When a user clicks on a zoom button the page calls itself with a different zoom setting in the url. The page works fine, but sometimes the map is cached in the browser so the user sees the old zoom level. I can't change the users caching settings, so can I do it in PHP? (I know in ASP there is a response.expires command - is there something similar I can use?) Thanks Nick