if ($user_id > 0 && $stmt = $conn->prepare("INSERT INTO chat (id, message, added) VALUES (?, ?, NOW())")) {
    $stmt->bind_param("is", $user_id, $_POST['msg']);
    $stmt->execute();
    $stmt->close(;)
} else {
    die("Could not insert new message. user_id: " . $user_id . ", $stmt || $conn errors: ". $stmt->error . " | " . $conn->error);
}