I'm trying to show alert like this https://i.stack.imgur.com/0lSAk.png
My test code looks like this
$url = "https://api.telegram.org/botTOKEN/answerCallbackQuery";
$response = file_get_contents($url."?callback_query_id=". $callback_query_id ."&text=SUCCESS&show_alert=true");
$response was { "ok": true, "result": true }
, so I think parameter $callback_query_id is correct.
But alert didn't appear. That is the problem.
A tried other bots (not mine) and other devices, and they showed alerts properly. So, I'm pretty sure problem is not with my device or telegram application.
I also have tried CURL and some Telegram Bot libraries. But it could not help me.