How can I make the browser see CSS and Javascript changes?

I found that if you append the last modified timestamp of the file onto the end of the URL the browser will request the files when it is modified. For example in PHP:

function urlmtime($url) {
   $parsed_url = parse_url($url);
   $path = $parsed_url['path'];

   if ($path[0] == "https://stackoverflow.com/") {
       $filename = $_SERVER['DOCUMENT_ROOT'] . "https://stackoverflow.com/" . $path;
   } else {
       $filename = $path;
   }

   if (!file_exists($filename)) {
       // If not a file then use the current time
       $lastModified = date('YmdHis');
   } else {
       $lastModified = date('YmdHis', filemtime($filename));
   }

   if (strpos($url, '?') === false) {
       $url .= '?ts=" . $lastModified;
   } else {
       $url .= "&ts=" . $lastModified;
   }

   return $url;
}

function include_css($css_url, $media="all') {
   // According to Yahoo, using link allows for progressive 
   // rendering in IE where as @import url($css_url) does not
   echo '<link rel="stylesheet" type="text/css" media="' .
        $media . '" href="' . urlmtime($css_url) . '">'."\n";
}

function include_javascript($javascript_url) {
   echo '<script type="text/javascript" src="' . urlmtime($javascript_url) .
        '"></script>'."\n";
}