if (isset($_POST['envoyer'])) {

    $sujet = $_POST['sujet'];
    $message = $_POST['message'];

    $sql = "SELECT username, email FROM users";
    $result = $pdo->query($sql);

    while ($row = $result->fetch()) {

        $nom = $row['username'];
        $email = $row['email'];

        $msg = "Bonjour $nom,\n\n$message\n\nWhine Dinning";
        echo "Envoyé à : $email <br>";
    }
}