Note that not all HTTP servers send the Content-Length header that is returned by head(). Most servers do, and a lot even do so if the actual content is generated dynamically. But some servers don't, especially for dynamic content. In that case, $document_length is undefined.

You can see this e.g. when downloading a file in Firefox: It always shows you the number of bytes downloaded so far and the time elapsed, and most of the times also the overall size of the download and the estimated remaining time. This happens when a Content-Length header was sent. Sometimes, Firefox does not tell you anything about the total size or the remaining time, simply because it does not know: There was no Content-Length header in the response.

Alexander