public function actionOauthLogin()
{
    $query = http_build_query([
        'client_id' => '12',
        'client_secret' => '',
        'redirect_uri' => 'http://client.loc/api/oauth/callback',
        'response_type' => 'code',
        'scope' => '',
    ]);
    return $this->redirect('http://oauth-server.loc/oauth/authorize?' . $query);
}

public function fomIdCallback()
{
    $http = new Client();
    $response = $http->post('http://oauth-server.loc/oauth/token', [
        'form_params' => [
            'grant_type' => 'authorization_code',
            'client_id' => '3',
            'client_secret' => 'TJDyfygkuga45rtyfj8&65567Yhhgjjjj',
            'redirect_uri' => 'http://client.loc/api/oauth/callback',
            'code' => Yii::app()->request->getParamFromRequest('code'),
        ],
    ]);
    return json_decode((string) $response->getBody(), true);
}

{"error":"invalid_client","message":"Client authentication failed"}