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Byte serving from PHP Razvan V. Florian
What is byte serving? Byte serving (or byteserving) is the ability of a web server to
provide a range of bytes in a file instead of the entire file. If a file is
being “byte served,” that means that the server which is sending the file is
able to give specific bytes that the client (e.g., a web browser) requests.
This feature of the HTTP protocol is commonly used by the Adobe Acrobat Reader
plugin. For example, if a PDF file is being byte served, Acrobat can ask for
the bytes for the 2nd page, and the server will send only the bytes for the
2nd page. In order to benefit of this HTTP protocol feature for viewing PDF
files, two conditions must be fulfilled: Why byte serving from PHP? The byte serving of regular PDF files is usually managed by the web server, if it is set up correspondingly. However, one may sometimes need to generate PDF file dynamically from PHP. For example, we would like to restrict access to certain PDF files to users authenticated by PHP, or to serve PDF files generated on the fly from PHP (e.g., using PDFLib or the R&OS PDF class). If we would like our users to benefit of byte serving (view the first page of the file, or other particular pages without downloading the entire file), we must implement byte serving from inside PHP. How to do it? To implement byteserving, the script should do the following: You may check below for a sample implementation. For more details
about how byteserving works, you may check the documentation for the HTTP/1.1
protocol. The script <?
/*
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);
$content_length+=strlen("\r\n--$boundary\r\n");
$content_length+=strlen("Content-type: application/pdf\r\n");
$content_length+=strlen("Content-range: bytes $first-$last/$filesize\r\n\r\n");
$content_length+=$last-$first+1;
}
$content_length+=strlen("\r\n--$boundary--\r\n");
//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, $last);
echo "\r\n--$boundary\r\n";
echo "Content-type: application/pdf\r\n";
echo "Content-range: bytes $first-$last/$filesize\r\n\r\n";
fseek($file,$first);
buffered_read ($file, $last-$first+1);
}
echo "\r\n--$boundary--\r\n";
} 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!
?>
Known problems It seems that when a byte ranges request by the Acrobat Reader plugin is not delivered by the server, Acrobat Reader may give a "There was a problem reading this document (109)" error when the user gets to the part of the PDF file that was not delivered. A typical scenario to get this error is to navigate quickly back and forth inside a PDF file while it downloads (by clicking the side navigation bar, for example). This results in many requests being submitted to the server in a short time, and the response to some of them may not reach the Acrobat Reader, due to network latency. However, this is not a bug of this script, but a bug in Acrobat: the same phenomenon also appears with files served directly by the Apache server. If you discover bugs in this script, or repeatable patterns of unexpected behavior, please report them. Further support If you would like further support on byte serving from PHP, or on other web development issues from me or my colleagues at Arxia, please write me. Acknowledgements I thank to Mathieu Roche and Gaetano Giunta for detecting some bugs in previous versions of the script. Last updated May 25, 2005; first released July 23, 2004. |
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