Guzzle HTTP client library added to replace drupal_http_request()

Is there a reason we couldn't have put Guzzle behind drupal_http_request?

The logic of including Guzzle is sound, but do we have to change the interface every time we introduce something new.

Update: Looks like the page has been updated to reflect this comment, thanks to ingaro.

The D8 code doesn't appear to work anymore. Looks like there's no longer a createRequest() method in Guzzle.

Per the Guzzle Documentation, this should work for the example above:

$client = \Drupal::httpClient();
$response = $client->request('GET', $feed->url, [
    'headers' => [
        'If-Modified-Since', gmdate(DATE_RFC1123, $last_fetched)
    ]
]);

try {
  $response = $client->send($request);
  // Expected result.
  $data = $response->getBody();
}
catch (RequestException $e) {
  watchdog_exception('my_module', $e->getMessage());
}

https://www.drupal.org/node/2691645 describes in detail.

As mentioned in one of the other comments, createRequest is no longer a thing. The same comment also suggests that the example has been updated, but actually they still mention createRequest? Is there a reason, or shall I jump in and change it up?
At the same time, I see that it might not make sense to keep a change log up to date with all the subsequent api changes... but the page is so chock-full of useful information for D7 people looking to do an http request in D8 that it would be good to either update or to put a big obvious link to the appropriate documentation.

The d8 code in the docs doesn't seem to work with D8.3, and I've been struggling to find
any examples of making more than a simple get request.

Here's an example of making a POST request, with http authentication, custom headers and
posting form data:

$client = \Drupal::httpClient();
$url = 'http://example.com/something';
$method = 'POST';
$options = [
  'auth' =>
    'username',
    'password',
  ],
 'headers' => [
    'X-My-Example-Header' => 'value',
  ],
  'form_params' => [
    'name' => 'value',
  ],
];
try {
  $response = $client->request($method, $url, $options);
  $code = $response->getStatusCode();
  if ($code == 200) {
    $body = $response->getBody()->getContents();
    return $body;
  }
}
catch (RequestException $e) {
  watchdog_exception('custom_modulename', $e);
}

Just wanted to point something out. Since the createRequest() method is no longer available, you can do the same thing this way:

use GuzzleHttp\Psr7\ServerRequest;

/* ... */

foreach($links as $url) {
  $requests[$url] = new ServerRequest('GET', $url); // replaces \Drupal::httpClient()->createRequest()
}

$batch_results = Pool::batch(\Drupal::httpClient(), $requests, ['http_errors' => FALSE]);

If you need to handle session (or other) cookies with the request, check the core test for an example
https://api.drupal.org/api/drupal/core%21modules%21system%21tests%21src%...

This is working for me, sending a XML file with \Drupal::httpClient() POST

$endpoint  = 'http://example.com/something';
$xml = '<>'; // Your XML here.

// Make the request.
$options = [
  'connect_timeout' => 30,
  'debug' => true,
  'headers' => array(
    'Content-Type' => 'text/xml',
  ),
  'body' => $xml,
  'verify'=>true,
];

try {
  $client = \Drupal::httpClient();
  $request = $client->request('POST',$endpoint,$options);

}
catch (RequestException $e){
  // Log the error.
  watchdog_exception('custom_modulename', $e);
}

$responseStatus = $request->getStatusCode();
$responseXml = $request->getBody()->getContents();

Hope this help.