$app->post("/message", function($params){
<?=var_dump($params) ?> // пусто
$config = array(
'base_uri' => 'https://api.vk.com/method/',
'query' => [
'user_id'=>'********',
'message' => $params,
'attachment' => '********',
'random_id' => rand(1E9, 9E9),
'access_token' => '********',
'v' => '5.59',
'verify' => false]);
$client = new \GuzzleHttp\Client($config);
$response = $client->get("messages.send");
return json_decode($response->getBody(), true);