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\Client
$client->get('..')/$client->post('..') will send the request directly, no longer need to call ->send().
Instead of $client->createRequest(..)->send(), you should call $client->sendRequest($client->createRequest(..)).
When sending an AJAX body, convert the body to a string with json_encode.
The response is a PSR-7 Response object. You can call $response->getBody()->getContents() to get the body as string.
$response->json() and $response->xml() are gone, but Omnipay\Common\Http\ResponseParser::json($response) and Omnipay\Common\Http\ResponseParser::xml($response) can be used instead.
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.

The $headers parameters should be an array (not null, but can be empty)
$body should be a string (eg. http_build_query($data) or json_encode($data) instead of just $data.