Upgrade apps from 2.x to 3.x

The redirect() method no calls exit() after sending the content. This is up to the developer now.

Upgrade Gateways from 2.x to 3.x

The primary difference is the HTTP Client. We are now using HTTPlug (http://httplug.io/) but rely on our own interface.

Change typehint from Guzzle ClientInterface to Omnipay\Common\Http\ClientInterface
$client->get('..')/$client->post('..') etc are removed, you can call $client->request('GET', '').
No need to call $request->send(), requests are sent directly.
Instead of $client->createRequest(..) you can create+send the request directly with $client->request(..).
When sending a JSON body, convert the body to a string with json_encode() and set the correct Content-Type.
The response is a PSR-7 Response object. You can call $response->getBody()->getContents() to get the body as string.
$response->json() and $response->xml() are gone, but you can implement the logic directly.
An HTTP Client is no longer added by default by omnipay/common, but omnipay/omnipay will add Guzzle. Gateways should not rely on Guzzle or other clients directly.
$body should be a string (eg. http_build_query($data) or json_encode($data) instead of just $data).
The $headers parameters should be an array (not null, but can be empty)

Note: Prior to stable release, the goal is to migrate to PSR-18, once completed. This will not change the gateway implementations.