How to return a file in PHP

I think you want this:

        $attachment_location = $_SERVER["DOCUMENT_ROOT"] . "/file.zip";
        if (file_exists($attachment_location)) {

            header($_SERVER["SERVER_PROTOCOL"] . " 200 OK");
            header("Cache-Control: public"); // needed for internet explorer
            header("Content-Type: application/zip");
            header("Content-Transfer-Encoding: Binary");
            header("Content-Length:".filesize($attachment_location));
            header("Content-Disposition: attachment; filename=file.zip");
            readfile($attachment_location);
            die();        
        } else {
            die("Error: File not found.");
        } 

readfile will do the job OK and pass the stream straight back to the webserver. It's not the best solution as for the time the file is sent, PHP still runs. For better results you'll need something like X-SendFile, which is supported on most webservers (if you install the correct modules).

In general (if you care about heavy load), it's best to put a proxying webserver in front of your main application server. This will free up your application server (for instance apache) up quicker, and proxy servers (Varnish, Squid) tend to be much better at transfering bytes to clients with high latency or clients that are generally slow.