I couldn't get some of the previous resume scripts to work with Free Download Manager or Firefox. I did some clean up and modified the code a little.
Changes:
1. Added a Flag to specify if you want download to be resumable or not
2. Some error checking and data cleanup for invalid/multiple ranges based on http://tools.ietf.org/id/draft-ietf-http-range-retrieval-00.txt
3. Always calculate a $seek_end even though the range specification says it could be empty... eg: bytes 500-/1234
4. Removed some Cache headers that didn't seem to be needed. (add back if you have problems)
5. Only send partial content header if downloading a piece of the file (IE workaround)
<?php
function dl_file_resumable($file, $is_resume=TRUE)
{
 //First, see if the file exists
 if (!is_file($file))
 {
 die("<b>404 File not found!</b>");
 }
 //Gather relevent info about file
 $size = filesize($file);
 $fileinfo = pathinfo($file);
 
 //workaround for IE filename bug with multiple periods / multiple dots in filename
 //that adds square brackets to filename - eg. setup.abc.exe becomes setup[1].abc.exe
 $filename = (strstr($_SERVER['HTTP_USER_AGENT'], 'MSIE')) ?
 preg_replace('/\./', '%2e', $fileinfo['basename'], substr_count($fileinfo['basename'], '.') - 1) :
 $fileinfo['basename'];
 
 $file_extension = strtolower($path_info['extension']);
 //This will set the Content-Type to the appropriate setting for the file
 switch($file_extension)
 {
 case 'exe': $ctype='application/octet-stream'; break;
 case 'zip': $ctype='application/zip'; break;
 case 'mp3': $ctype='audio/mpeg'; break;
 case 'mpg': $ctype='video/mpeg'; break;
 case 'avi': $ctype='video/x-msvideo'; break;
 default: $ctype='application/force-download';
 }
 //check if http_range is sent by browser (or download manager)
 if($is_resume && isset($_SERVER['HTTP_RANGE']))
 {
 list($size_unit, $range_orig) = explode('=', $_SERVER['HTTP_RANGE'], 2);
 if ($size_unit == 'bytes')
 {
 //multiple ranges could be specified at the same time, but for simplicity only serve the first range
 //http://tools.ietf.org/id/draft-ietf-http-range-retrieval-00.txt
 list($range, $extra_ranges) = explode(',', $range_orig, 2);
 }
 else
 {
 $range = '';
 }
 }
 else
 {
 $range = '';
 }
 //figure out download piece from range (if set)
 list($seek_start, $seek_end) = explode('-', $range, 2);
 //set start and end based on range (if set), else set defaults
 //also check for invalid ranges.
 $seek_end = (empty($seek_end)) ? ($size - 1) : min(abs(intval($seek_end)),($size - 1));
 $seek_start = (empty($seek_start) || $seek_end < abs(intval($seek_start))) ? 0 : max(abs(intval($seek_start)),0);
 //add headers if resumable
 if ($is_resume)
 {
 //Only send partial content header if downloading a piece of the file (IE workaround)
 if ($seek_start > 0 || $seek_end < ($size - 1))
 {
 header('HTTP/1.1 206 Partial Content');
 }
 header('Accept-Ranges: bytes');
 header('Content-Range: bytes '.$seek_start.'-'.$seek_end.'/'.$size);
 }
 //headers for IE Bugs (is this necessary?)
 //header("Cache-Control: cache, must-revalidate"); 
 //header("Pragma: public");
 header('Content-Type: ' . $ctype);
 header('Content-Disposition: attachment; filename="' . $filename . '"');
 header('Content-Length: '.($seek_end - $seek_start + 1));
 //open the file
 $fp = fopen($file, 'rb');
 //seek to start of missing part
 fseek($fp, $seek_start);
 //start buffered download
 while(!feof($fp))
 {
 //reset time limit for big files
 set_time_limit(0);
 print(fread($fp, 1024*8));
 flush();
 ob_flush();
 }
 fclose($fp);
 exit;
}
?>