How do you log all API calls using Guzzle 6

You can use any logger which implements PSR-3 interface with Guzzle 6

I used Monolog as logger and builtin middleware of Guzzle with MessageFormatter in below example.

use GuzzleHttp\HandlerStack;
use GuzzleHttp\Middleware;
use GuzzleHttp\MessageFormatter;
use Monolog\Logger;

$stack = HandlerStack::create();
$stack->push(
    Middleware::log(
        new Logger('Logger'),
        new MessageFormatter('{req_body} - {res_body}')
    )
);
$client = new \GuzzleHttp\Client(
    [
        'base_uri' => 'http://httpbin.org',
        'handler' => $stack,
    ]
);

echo (string) $client->get('ip')->getBody();

The details about the log middleware and message formatter has not well documented yet. But you can check the list which variables you can use in MessageFormatter

Also there is a guzzle-logmiddleware which allows you to customize formatter etc.

@KingKongFrog This is the way to specify the name of the log file

$logger = new Logger('MyLog');
$logger->pushHandler(new StreamHandler(__DIR__ . '/test.log'), Logger::DEBUG);

$stack->push(Middleware::log(
$logger,
new MessageFormatter('{req_body} - {res_body}')
));