Streaming POST data through PHP cURL Using CURLOPT_READFUNCTION

Well, I haven’t posted here in quite some time… I’m not dead, and don’t plan on completely ditching this blog, but well…

Anyway, onto the article.

I had a PHP application where I wanted to upload part of a large file to some other server.  The naive method may be to simply split the file and upload through cURL, however I wanted to do this without any splitting.  So I needed a way to send a POST request, being able to build the request body on the fly (note, you’ll need to know the total size to be able to send the Content-Length header)

The obvious decision would be to use sockets rather than cURL, but I felt like seeing if it was possible with cURL anyway.  Although I’ll still probably use sockets (because it’s easier in the end), I thought this might (well, not really) be useful to one of the three readers I get every month.

Anyway, if you look at the curl_setopt documentation, you’ll see a CURLOPT_READFUNCTION constant, however, how to really use it doesn’t seem clear (especially with the boundaries for multipart/form-data encoding type).  Also, the documentation is wrong.

Without further ado, here’s some sample code:


$boundary = '-----------------------------168279961491';
// our request body
$str = "$boundary\r\nContent-Disposition: form-data; name='how_do_i_turn_you'\r\n\r\non\r\n$boundary--\r\n";

// set up cURL
curl_setopt_array($ch, array(
 CURLOPT_POST => true,
 CURLOPT_HTTPHEADER => array( // we need to send these two headers
 'Content-Type: multipart/form-data; boundary='.$boundary,
 'Content-Length: '.strlen($str)
 // note, do not set the CURLOPT_POSTFIELDS setting

// function to stream data
// I'm not sure what the file pointer $fp does in this context
// but $ch is the cURL resource handle, and $len is how many bytes to read
function myfunc($ch, $fp, $len) {
 static $pos=0; // keep track of position
 global $str;
 // set data
 $data = substr($str, $pos, $len);
 // increment $pos
 $pos += strlen($data);
 // return the data to send in the request
 return $data;

// execute request, and show output for lolz
echo curl_exec($ch);

Hopefully the comments give you enough idea how it all works.

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