XML-RPC failing to respond to POST requests via cURL in PHP - php

Question

I'm having some issues with calling WordPress XML-RPC via cURL in PHP. It's a WordPress.com hosted blog, and the XML-RPC file is located at http://sunseekerblogbook.com/xmlrpc.php.
Starting yesterday (or at least, yesterday was when it was noticed), cURL has been failing with error #52: Empty reply from server.
The code snippet we're using is below:
$ch = curl_init('http://sunseekerblogbook.com/xmlrpc.php');
curl_setopt_array($ch, [
CURLOPT_HEADER => false,
CURLOPT_HTTPHEADER => [
'Content-Type: text/xml'
],
CURLOPT_POSTFIELDS => xmlrpc_encode_request('wp.getPosts', [
1,
WP_USERNAME,
WP_PASSWORD,
[
'number' => 15
]
]),
CURLOPT_RETURNTRANSFER => true
]);
$ret = curl_exec($ch);
$data = xmlrpc_decode($ret, 'UTF-8');
Using cURL directly however, everything returns exactly as expected:
$output = [];
exec('curl -d "<?xml version=\"1.0\" encoding=\"UTF-8\"?><methodCall><methodName>wp.getPosts</methodName><params><param><value><int>1</int></value></param><param><value><string>' . WP_USERNAME . '</string></value></param><param><value><string>' . WP_PASSWORD . '</string></value></param><param><value><struct><member><name>number</name><value><int>15</int></value></member></struct></value></param></params></methodCall>" sunseekerblogbook.com/xmlrpc.php', $output);
$data = xmlrpc_decode(implode('', $output), 'UTF-8');
We've been successfully able to query WordPress since July 2013, and we're at a dead-end as to why this has happened. It doesn't look like PHP or cURL have been updated/changed recently on the server, but the first code snippet has failed on every server we've tried it on now (with PHP 5.4+).
Using the http://sunseekerblogbook.wordpress.com/xmlrpc.php link gives the same issue.
Is there anything missing from the PHP code that would cause this issue? That it's suddenly stopped working over 12 months down the line is what has flummoxed me.

Answer 1

Managed to fix it. Looking at the headers sent by cURL, the only differences were that the cURL command line uses Content-Type: application/x-www-form-urlencoded and that the user agent was set to User-Agent: curl/7.30.0.
The choice of content type didn't affect it, but setting a user agent sorted it! It seems WordPress.com (but not self-hosted WordPress.org sites running the latest v3.9.2) now requires a user agent for XML-RPC requests, though this hasn't been documented anywhere that I can find.