log all outgoing requests using guzzle's handlers

composer.json

{
    "require": {
        "guzzlehttp/guzzle": "^6.5",
        "monolog/monolog": "^1.25"
    }
}

index.php

<?php

require __DIR__ . '/vendor/autoload.php';

use GuzzleHttp\Client;
use GuzzleHttp\HandlerStack;
use Monolog\Logger;
use Monolog\Handler\StreamHandler;

$logger = new Logger('responses'); //initialize the logger
$logger->pushHandler(new StreamHandler(__DIR__ . '/dist/text.log', Logger::DEBUG));

$handler = HandlerStack::create();
$handler->push(new LoggingMiddleware($logger), 'logger');

$client  = new Client(['handler' => $handler]); //initialize a Guzzle client

$response = $client->post('https://postman-echo.com/post', [ // use postman test service to send a request
	'headers' => [
		'content-type' => 'application/json'
	],
	'body' => json_encode(['foo' => 'bar'])
]);

LoggingMiddleware.php

<?php

use Psr\Http\Message\RequestInterface;
use Psr\Http\Message\ResponseInterface;
use Psr\Log\LoggerInterface;

class LoggingMiddleware
{
	/** @var LoggerInterface */
	private $logger;

	/**
	 * construct
	 *
	 * @param LoggerInterface $logger
	 */
	public function __construct(LoggerInterface $logger)
	{
		$this->logger = $logger;
	}

	/**
	 * invoke class
	 *
	 * @param callable $handler
	 * @return callable
	 */
	public function __invoke(callable $handler)
	{
		return function (RequestInterface $request, $options) use ($handler) { // return a callable
			$promise = $handler($request, $options); // get the new handler

			return $promise->then(function (ResponseInterface $response) use ($request) { // add a new promise which is a callback after response has been received
				$this->logger->info(print_r([
					'method' => (string) $request->getMethod(),
					'url' => (string) $request->getUri(),
					'response' => (string) $response->getBody(),
					'request' => (string) $request->getBody()
				], true));

				return $response;
			});
		};
	}
}