README.md

composer require mailgun/mailgun-php

// ...

$app->register(new MailgunServiceProvider());

$app['mailgun.options'] = [
    'api_key' => '',
    'domain' => ''
];

$app['mailgun']->sendMessage($app['mailgun.options']['domain'], [
    'from' => '',
    'to'  => '',
    'subject' => '',
    'text' => ''
]);

MailgunServiceProvider.php

<?php

namespace Sergiors\Sergiors\Silex\Provider;

use Silex\Application;
use Silex\ServiceProviderInterface;
use Mailgun\Mailgun;
//use Pimple/Container;
//use Pimple/ServiceProviderInterface;


class MailgunServiceProvider implements ServiceProviderInterface
{
    //public function register(Container $app)
    public function register(Application $app)
    {
        $app['mailgun'] = $app->share(function (Application $app) {
            return new Mailgun($app['mailgun.options']['api_key']);
        });

        $app['mailgun.options'] = [
            'api_key' => null,
            'domain' => null
        ];
    }
    
    public function boot(Application $app)
    {
    }
}