Before middleware to load Guzzle 6.

Controller.php

<?php

use Silex\Application;
use Silex\ControllerProviderInterface;

/**
 * Controller.
 *
 * @author Gawain Lynch <gawain.lynch@gmail.com>
 */
class Controller implements ControllerProviderInterface
{
    /**
     * @param \Silex\Application $app
     *
     * @return \Silex\ControllerCollection
     */
    public function connect(Application $app)
    {
        /** @var $ctr \Silex\ControllerCollection */
        $ctr = $app['controllers_factory']
            ->before([$this, 'before']);

        $ctr->match('/', [$this, 'myCallback'])
            ->method('GET|POST');

        return $ctr;
    }

    /**
     * Before middleware to load Guzzle 6.
     */
    public function before()
    {
        $baseDir = dirname(dirname(__DIR__));

        require $baseDir . '/lib/GuzzleHttp/Guzzle/functions_include.php';
        require $baseDir . '/lib/GuzzleHttp/Promise/functions_include.php';
        require $baseDir . '/lib/GuzzleHttp/Psr7/functions_include.php';

        $loader = new \Composer\Autoload\ClassLoader();
        $loader->setPsr4('GuzzleHttp\\', [
            $baseDir . '/lib/GuzzleHttp/Guzzle',
            $baseDir . '/lib/GuzzleHttp/Promise',
            $baseDir . '/lib/GuzzleHttp/Psr7',
        ]);
        $loader->register(true);
    }
}