“Transfer-Encoding: chunked” header in PHP. What is "padding" for?

Question

As suggested in a reply to a previous question (PHP External Oauth : how to displaying a waiting message while waiting for callback (not using AJAX) ), I am using transfer encoding: chunked to display a waiting message while some tasks are performed. My first attempts failed, and I found a solution in this question “Transfer-Encoding: chunked” header in PHP. There is a "padding" of 1024 blank space. Without this padding it doesn't work. I have googled around but I can't find what this padding is for. Here is the sample code (from the related question).

<?php
        header('Content-Encoding', 'chunked');
        header('Transfer-Encoding', 'chunked');
        header('Content-Type', 'text/html');
        header('Connection', 'keep-alive');

        ob_flush();
        flush();

        $p = "";  //padding
        for ($i=0; $i < 1024; $i++) { 
            $p .= " ";
        };
        echo $p;

        ob_flush();
        flush();

        for ($i = 0; $i < 10000; $i++) {
            echo "string";
            ob_flush();
            flush();
            sleep(2);
        }

?>

Does anybody have an explanation why it works with and doesn't work without the "padding" ?

Answer 1

The padding is for filling the server buffer, as I understand.

Without it the server will wait until PHP will fill it and after this will flush it - even if in PHP code you do flush().

Related:

• PHP Output buffer flush issue on Apache/Linux (May 2010; on Server Fault)

Answer 2

I have no idea what this padding is supposed to do, and actually it shouldn't work (someone may enlighten me if I'm wrong on this). The idea with chunked encoding is that you send your data in chunks. Each chunk consists of a line containing the length of a chunk, followed by a newline and then the data of the chunk. A response can contain as many chunks as you want to have. So basically the response of 3 chunks containing "Hello" would look like this:

5 <--- this is the length of the chunk, that is "Hello" == 5 chars
Hello  <--- This is a the actual data
<-- an empty line is between the chunks
5
Hello

5
Hello

<-- send two empty lines to end the transmission

So I'd rewrite this to something like:

<?php
        header('Content-Encoding', 'chunked');
        header('Transfer-Encoding', 'chunked');
        header('Content-Type', 'text/html');
        header('Connection', 'keep-alive');

        ob_flush();
        flush();

        for ($i = 0; $i < 10000; $i++) {
            $string = "string";
            echo strlen($string)."\r\n"; // this is the length
            echo $string."\r\n"; // this is the date
            echo "\r\n"; // newline between chunks
            ob_flush(); // rinse and repeat
            flush();
            sleep(2);
        } 
        echo  "\r\n"; // send final empty line
        ob_flush();
        flush();

?>

The code above will not work at all circumstances (e.g. with strings containing newlines or non-ascii encodings), so you will have to adapt it to your use case.