<?php
function force_download ($data, $name, $mimetype='', $filesize=false) {
    // File size not set?
    if ($filesize == false OR !is_numeric($filesize)) {
        $filesize = strlen($data);
    }

    // Mimetype not set?
    if (empty($mimetype)) {
        $mimetype = 'application/octet-stream';
    }

    // Make sure there's not anything else left
    ob_clean_all();

    // Start sending headers
    header("Pragma: public"); // required
    header("Expires: 0");
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
    header("Cache-Control: private",false); // required for certain browsers
    header("Content-Transfer-Encoding: binary");
    header("Content-Type: " . $mimetype);
    header("Content-Length: " . $filesize);
    header("Content-Disposition: attachment; filename=\"" . $name . "\";" );

    // Send data
    echo $data;
    die();
}

function ob_clean_all () {
    $ob_active = ob_get_length () !== false;
    while($ob_active) {
        ob_end_clean();
        $ob_active = ob_get_length () !== false;
    }

    return true;
}
?>

<a href="http://www.mysite.com/files/this_thing_i_made.zip">download this thing I made</a>

http://ftp.blizzard.com/pub/WoW/other/InterfaceAddOnKit_Win.zip