google analytics provider (gtag) for silex 1.x

DMG\ServiceProvider\GoogleAnalyticsProvider.php

<?php

namespace DMG\ServiceProvider;

use Silex\Application;
use Silex\ServiceProviderInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

/**
 * Class GoogleAnalyticsProvider
 *
 * @package DMG\ServiceProvider
 */
class GoogleAnalyticsProvider implements ServiceProviderInterface
{
	/** @var string */
	protected $ua;

	public function __construct($ua)
	{
		$this->ua = $ua;
	}

	/**
	 * @param \Silex\Application $app
	 */
	public function boot(Application $app)
	{
		$app->after(array($this, 'inject'));
	}

	/**
	 * @param \Silex\Application $app
	 */
	public function register(Application $app)
	{
	}

	public function inject(Request $request, Response $response)
	{
		if ($response->getStatusCode() == 200 &&
		    false === strpos($request->get('_route'), 'cms.') &&
		    false !== stripos($response->headers->get('Content-Type'), 'html') &&
		    false !== stripos($content = $response->getContent(), '</head>')
		) {
			$method     = __METHOD__;
			$ua         = $this->ua;
			$ua_encoded = json_encode($this->ua);
			$response->setContent(preg_replace('@<head(\s[^>]+)?>@i',
				'${0}'."\n\t<!-- Global site tag (gtag.js) - Google Analytics -->
	<script async src=\"https://www.googletagmanager.com/gtag/js?id=$ua\"></script>
	<script>
	  window.dataLayer = window.dataLayer || [];
	  function gtag(){dataLayer.push(arguments);}
	  gtag('js', new Date());

	  gtag('config', $ua_encoded);
	</script>\n",
				$content));
		}
	}
}