Is there a good implementation of partial file downloading in PHP?

I’m writing a PHP script that allows the user to download a file. Basically the idea is to prevent the file being downloaded more than X times, since it is paid content, and the link should not be spread around.

Since the files will be pretty large, it should be good to implement resuming. I’ve read the standard, but it’s pretty long and allows for some flexibility. Since I need to get it done quickly, I’d prefer a stable, tested implementation of this feature.

Can anyone point me to such a a script?

Answer

Seems that I found what I needed myself. So that other may benefit from this, here is the link: http://www.coneural.org/florian/papers/04_byteserving.php

And just in case the original page stops to work (the script is pretty old already), here is a copy of it:

<?php 
/*

The following byte serving code is (C) 2004 Razvan Florian. You may find the latest version at 
http://www.coneural.org/florian/papers/04_byteserving.php

*/
function set_range($range, $filesize, &$first, &$last){
  /*
  Sets the first and last bytes of a range, given a range expressed as a string 
  and the size of the file.

  If the end of the range is not specified, or the end of the range is greater 
  than the length of the file, $last is set as the end of the file.

  If the begining of the range is not specified, the meaning of the value after 
  the dash is "get the last n bytes of the file".

  If $first is greater than $last, the range is not satisfiable, and we should 
  return a response with a status of 416 (Requested range not satisfiable).

  Examples:
  $range='0-499', $filesize=1000 => $first=0, $last=499 .
  $range='500-', $filesize=1000 => $first=500, $last=999 .
  $range='500-1200', $filesize=1000 => $first=500, $last=999 .
  $range='-200', $filesize=1000 => $first=800, $last=999 .

  */
  $dash=strpos($range,'-');
  $first=trim(substr($range,0,$dash));
  $last=trim(substr($range,$dash+1));
  if ($first=='') {
    //suffix byte range: gets last n bytes
    $suffix=$last;
    $last=$filesize-1;
    $first=$filesize-$suffix;
    if($first<0) $first=0;
  } else {
    if ($last=='' || $last>$filesize-1) $last=$filesize-1;
  }
  if($first>$last){
    //unsatisfiable range
    header("Status: 416 Requested range not satisfiable");
    header("Content-Range: */$filesize");
    exit;
  }
}

function buffered_read($file, $bytes, $buffer_size=1024){
  /*
  Outputs up to $bytes from the file $file to standard output, $buffer_size bytes at a time.
  */
  $bytes_left=$bytes;
  while($bytes_left>0 && !feof($file)){
    if($bytes_left>$buffer_size)
      $bytes_to_read=$buffer_size;
    else
      $bytes_to_read=$bytes_left;
    $bytes_left-=$bytes_to_read;
    $contents=fread($file, $bytes_to_read);
    echo $contents;
    flush();
  }
}

function byteserve($filename){
  /*
  Byteserves the file $filename.  

  When there is a request for a single range, the content is transmitted 
  with a Content-Range header, and a Content-Length header showing the number 
  of bytes actually transferred.

  When there is a request for multiple ranges, these are transmitted as a 
  multipart message. The multipart media type used for this purpose is 
  "multipart/byteranges".
  */

  $filesize=filesize($filename);
  $file=fopen($filename,"rb");

  $ranges=NULL;
  if ($_SERVER['REQUEST_METHOD']=='GET' && isset($_SERVER['HTTP_RANGE']) && $range=stristr(trim($_SERVER['HTTP_RANGE']),'bytes=')){
    $range=substr($range,6);
    $boundary='g45d64df96bmdf4sdgh45hf5';//set a random boundary
    $ranges=explode(',',$range);
  }

  if($ranges && count($ranges)){
    header("HTTP/1.1 206 Partial content");
    header("Accept-Ranges: bytes");
    if(count($ranges)>1){
      /*
      More than one range is requested. 
      */

      //compute content length
      $content_length=0;
      foreach ($ranges as $range){
        set_range($range, $filesize, $first, $last);
        $content_length+=strlen("rn--$boundaryrn");
        $content_length+=strlen("Content-type: application/pdfrn");
        $content_length+=strlen("Content-range: bytes $first-$last/$filesizernrn");
        $content_length+=$last-$first+1;          
      }
      $content_length+=strlen("rn--$boundary--rn");

      //output headers
      header("Content-Length: $content_length");
      //see http://httpd.apache.org/docs/misc/known_client_problems.html for an discussion of x-byteranges vs. byteranges
      header("Content-Type: multipart/x-byteranges; boundary=$boundary");

      //output the content
      foreach ($ranges as $range){
        set_range($range, $filesize, $first, $last);
        echo "rn--$boundaryrn";
        echo "Content-type: application/pdfrn";
        echo "Content-range: bytes $first-$last/$filesizernrn";
        fseek($file,$first);
        buffered_read ($file, $last-$first+1);          
      }
      echo "rn--$boundary--rn";
    } else {
      /*
      A single range is requested.
      */
      $range=$ranges[0];
      set_range($range, $filesize, $first, $last);  
      header("Content-Length: ".($last-$first+1) );
      header("Content-Range: bytes $first-$last/$filesize");
      header("Content-Type: application/pdf");  
      fseek($file,$first);
      buffered_read($file, $last-$first+1);
    }
  } else{
    //no byteserving
    header("Accept-Ranges: bytes");
    header("Content-Length: $filesize");
    header("Content-Type: application/pdf");
    readfile($filename);
  }
  fclose($file);
}

function serve($filename, $download=0){
  //Just serves the file without byteserving
  //if $download=true, then the save file dialog appears
  $filesize=filesize($filename);
  header("Content-Length: $filesize");
  header("Content-Type: application/pdf");
  $filename_parts=pathinfo($filename);
  if($download) header('Content-disposition: attachment; filename='.$filename_parts['basename']);
  readfile($filename);
}

//unset magic quotes; otherwise, file contents will be modified
set_magic_quotes_runtime(0);

//do not send cache limiter header
ini_set('session.cache_limiter','none');


$filename='myfile.pdf'; //this is the PDF file that will be byteserved
byteserve($filename); //byteserve it!
?>