Laravel 5.3 has this feature. But for those not using that version or who just want to see how easy this is, I will show here how I been doing this for some time now.

Here is the main Class I call to send a message. As you can see I am using Guzzle, in this case "guzzlehttp/guzzle": "^6.1" but others versions can work.

<?php
namespace App\Services;

use Illuminate\Support\Facades\Log;
use GuzzleHttp\Client;

class SlackWrapper
{
    /**
     * @var Client $client
     */
    protected $client;

    protected $slack_url = false;

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

    public function sendMessageToSlack($message)
    {
        try {
            $this->client->request(
                'POST',
                $this->getSlackUrl(),
                [
                    'body' => $this->message($message)
                ]
            );
        } catch (\Exception $e) {
            Log::debug(sprintf("Error sending to Slack %s", $e->getMessage()));
        }
    }

    protected function message($message)
    {
        return json_encode(
            [
                'text' => $message
            ]
        );
    }

    public function getSlackUrl()
    {
        if (!$this->slack_url)
            return env('SLACK_URL');
        return $this->slack_url;
    }

    /**
     * @param boolean $slack_url
     */
    public function setSlackUrl($slack_url)
    {
        $this->slack_url = $slack_url;
        return $this;
    }
}

And that is it. As long as I set this env('SLACK_URL') in my .env OR in the class instantiation it will get to that endpoint.

Then in slack just add get the webhook/room you want to post to