Requesting Tokens
Route::get('/redirect', function (Request $request) {
  $request->session()->put('state', $state = Str::random(40));
  $query = http_build_query([
      'client_id' => 'client-id',
      'redirect_uri' => 'http://example.com/callback',
      'response_type' => 'code',
      'scope' => '',
      'state' => $state,
  ]);
  return redirect('http://your-app.com/oauth/authorize?'.$query);
});

Converting Authorization Codes To Access Tokens
Route::get('/callback', function (Request $request) {
    $state = $request->session()->pull('state');

    throw_unless(
        strlen($state) > 0 && $state === $request->state,
        InvalidArgumentException::class
    );

    $http = new GuzzleHttp\Client;

    $response = $http->post('http://your-app.com/oauth/token', [
        'form_params' => [
            'grant_type' => 'authorization_code',
            'client_id' => 'client-id',
            'client_secret' => 'client-secret',
            'redirect_uri' => 'http://example.com/callback',
            'code' => $request->code,
        ],
    ]);

    return json_decode((string) $response->getBody(), true);
});

Retrieving Tokens
$guzzle = new GuzzleHttp\Client;
$response = $guzzle->post('http://your-app.com/oauth/token', [
    'form_params' => [
        'grant_type' => 'client_credentials',
        'client_id' => 'client-id',
        'client_secret' => 'client-secret',
        'scope' => 'your-scope',
    ],
]);
return json_decode((string) $response->getBody(), true)['access_token'];

Passing The Access Token
$response = $client->request('GET', '/api/user', [
    'headers' => [
        'Accept' => 'application/json',
        'Authorization' => 'Bearer '.$accessToken,
    ],
]);

JSON response

  {
      "token_type": "Bearer",
      "expires_in": 31536000,
      "access_token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImp0aSI6IjUjU7ucA...",
      "refresh_token": "def502007fafdd6b26b6bb7190f5f4dc4d2c93c6dd645249a55e..."
  }

User response

  {
      "id": 1,
      "name": "phanuwat",
      "email": "mr.phanuwat@gmail.com",
      "email_verified_at": null,
      "created_at": "2020-01-30 14:51:23",
      "updated_at": "2020-01-30 14:51:23"
  }