Send file with HTTPRange support (partial download):
<?php
function smartReadFile($location, $filename, $mimeType='application/octet-stream')
{ if(!file_exists($location))
  { header ("HTTP/1.0 404 Not Found");
    return;
  }
  
  $size=filesize($location);
  $time=date('r',filemtime($location));
  
  $fm=@fopen($location,'rb');
  if(!$fm)
  { header ("HTTP/1.0 505 Internal server error");
    return;
  }
  
  $begin=0;
  $end=$size;
  
  if(isset($_SERVER['HTTP_RANGE']))
  { if(preg_match('/bytes=\h*(\d+)-(\d*)[\D.*]?/i', $_SERVER['HTTP_RANGE'], $matches))
    { $begin=intval($matches[0]);
      if(!empty($matches[1]))
        $end=intval($matches[1]);
    }
  }
  
  if($begin>0||$end<$size)
    header('HTTP/1.0 206 Partial Content');
  else
    header('HTTP/1.0 200 OK');  
  
  header("Content-Type: $mimeType"); 
  header('Cache-Control: public, must-revalidate, max-age=0');
  header('Pragma: no-cache');  
  header('Accept-Ranges: bytes');
  header('Content-Length:'.($end-$begin));
  header("Content-Range: bytes $begin-$end/$size");
  header("Content-Disposition: inline; filename=$filename");
  header("Content-Transfer-Encoding: binary\n");
  header("Last-Modified: $time");
  header('Connection: close');  
  
  $cur=$begin;
  fseek($fm,$begin,0);
  while(!feof($fm)&&$cur<$end&&(connection_status()==0))
  { print fread($fm,min(1024*16,$end-$cur));
    $cur+=1024*16;
  }
}
?>
Usage:
<?php
smartReadFile("/tmp/filename","myfile.mp3","audio/mpeg")
?>
It can be slow for big files to read by fread, but this is a single way to read file in strict bounds. You can modify this and add fpassthru instead of fread and while, but it sends all data from begin --- it would be not fruitful if request is bytes from 100 to 200 from 100mb file.