If you use fork, and build the zip file with the child process, then the parent process can redirect to a "status" page. The status page will either display "working..." or "not done yet" while the zip is being built, or else display a clickable link to the zip once it's done. The ajax/javascript version of the page will automatically refresh the status every few seconds. If javascript is not available, you could try using a meta tag refresh, or else instruct users to click "reload" periodically (or put a message like "check back later" inside a noscript tag).