src/Messenger.php
public function listen()
    {
        if (!empty($_REQUEST['hub_verify_token']) && $_REQUEST['hub_verify_token'] === $this->verifyToken) {
            echo $_REQUEST['hub_challenge'];
            exit;
        }

        return json_decode(file_get_contents('php://input'), true);
    }
public function listen()
    {
        if (!empty($_REQUEST['hub_verify_token']) && $_REQUEST['hub_verify_token'] === $this->verifyToken) {
            echo $_REQUEST['hub_challenge'];
            exit;
        }

        return json_decode(file_get_contents('php://input'), true);
    }
Listen message

@return array