Php Curl Multipart form posting

Let's say you want to post some data to a form using curl:

$url = 'http://www.example.com/';
$data = array('foo' => '1', 'bar' => '2');

Ordinarily you create the post body using http_build_query(). But let's say the form is expecting the form data to be multipart encoded. Now we've got a challenge. First let's create a function to do the multipart encoding:

function multipart_build_query($fields, $boundary){
  $retval = '';
  foreach($fields as $key => $value){
    $retval .= "--$boundary\nContent-Disposition: form-data; name=\"$key\"\n\n$value\n";
  }
  $retval .= "--$boundary--";
  return $retval;
}

The boundary is a string that separates the fields. It can be any string you want but you should choose something long enough that it won't randomly show up in your data.

$boundary = '--myboundary-xxx';
$body = multipart_build_query($data, $boundary);

Now make your curl post, but remember to set the content type:

$ch = curl_init();
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: multipart/form-data; boundary=$boundary"));
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $body);
$response = curl_exec($ch);

Of course, you could have just used PGBrowser to submit the form, and it will automatically detect when it needs to use multipart encoding. But that would be too easy.