[WordPress][Backlog API] WordPress から xmlrpc クライアントをしようして、backlog API を叩く

gistfile1.php

<?php
include_once( ABSPATH . WPINC . '/class-IXR.php' );
include_once( ABSPATH . WPINC . '/class-wp-http-ixr-client.php');

add_filter( 'http_request_args', function( $args, $url ) {
  if ( false !== strpos($url, 'example.backlog.jp') ) {
    $args['headers']['Authorization'] = 'Basic '. base64_encode('backlog_user' . ':' . 'backlog_passwd');
  }
  return $args;
}, 10, 2 );

$ixr_client = new WP_HTTP_IXR_Client( 'https://example.backlog.jp/XML-RPC' );
if ( !$ixr_client->query('backlog.getProjects') ) {
  var_dump($ixr_client->getErrorCode());
}
print_r($ixr_client->getResponse());