Example Laravel AppServiceProvider class that consumes tagged Guzzle 6 middleware

AppServiceProvider.php

<?php

namespace App\Providers;

use App\HMACRequestHandler;
use GuzzleHttp\Client;
use GuzzleHttp\Handler\CurlHandler;
use GuzzleHttp\HandlerStack;
use Illuminate\Support\ServiceProvider;

class AppServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        //
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        $this->app->bind(HMACRequestHandler::class, function () {
            return new HMACRequestHandler('key', 'secret');
        });

        $this->app->tag([
            HMACRequestHandler::class,
        ], 'example.client.middleware');

        $this->app->singleton('example.client', function () {
            $handler = new CurlHandler();
            $stack = HandlerStack::create($handler);

            foreach ($this->app->tagged('example.client.middleware') as $middleware) {
                $stack->push($middleware);
            }

            return new Client(['handler' => $stack]);
        });
    }
}