Airbrake integration for CakePHP 3.x - To keep the CakePHP error flow.

config\bootstrap.php

<?php

// ...

use App\Error\AirbrakeHandler;

// ...

/**
 * Register application error and exception handlers.
 */
$isCli = PHP_SAPI === 'cli';
if ($isCli) {
    (new ConsoleErrorHandler(Configure::read('Error')))->register();
} else {
    (new AirbrakeHandler(Configure::read('Error')))->register();
}

src\Error\AirbrakeHandler.php

<?php
namespace App\Error;

use Cake\Core\Configure;
use Cake\Error\ErrorHandler;

use Airbrake\Notifier as AirbrakeNotifier;
use Airbrake\ErrorHandler as AirbrakeErrorHandler;
use Airbrake\Instance as AirbrakeInstance;

/**
 * Airbrake Handler, this class allows for an error or exception to be sent to airbrake, 
 * while also allowing the normal CakePHP error flow
 */
class AirbrakeHandler extends ErrorHandler
{
    protected $_airbrakeHandler;

    /**
     * Constructor
     *
     * @param array $options The options for error handling.
     */
    public function __construct($options = [])
    {
        $this->setupAirbrake();

        $options['debug']  = Configure::read('debug');
        parent::__construct($options);
    }

    /**
     * Setup airbrake instance and register shutdown function
     */
    protected function setupAirbrake()
    {
        // Create new Notifier instance.
        $notifier = new Airbrake\Notifier([
          'projectId' => 12345, // FIX ME
          'projectKey' => 'abcdefg' // FIX ME
        ]);

        // Set global notifier instance.
        AirbrakeInstance::set($notifier);

        // Setup the Airbrake Error Handler.
        $this->_airbrakeHandler = new AirbrakeErrorHandler($notifier);

        // Register shutdown function manually, since we're not calling $handler->register();
        register_shutdown_function([$this->_airbrakeHandler, 'onShutdown']);
    }

    /**
     * {@inheritDoc}
     */
    public function handleError($code, $description, $file = null, $line = null, $context = null)
    {
        $this->_airbrakeHandler->onError($code, $description, $file, $line);
        return parent::handleError($code, $description, $file, $line, $context);
    }

    /**
     * {@inheritDoc}
     */
    public function wrapAndHandleException($exception)
    {
        $this->_airbrakeHandler->onException($exception);
        parent::wrapAndHandleException($exception);
    }
}

I had to use AirbrakeNotifier rather than Airbrake\Notifier on line 38.