How to send a message to a user by @username and not by chat_id?

Question

PHP

I have a website with a form on it, you need a person to enter his @username in Telegram, and the bot sends him a message with a secret key. How to implement it via API?

Answer 1

1. You cannot send a message to a person on behalf of a bot if he has not interacted with him before.
2. Bot Api does not allow sending a message to a person by username, the exception is the channel.
https://core.telegram.org/bots/api#sendmessage chat_id
parameter :

Unique identifier for the target chat or username of the target channel (in the format @channelusername)

If the user has already communicated with the bot, then you can store the username and chat_id in the database, when entering in the username form, take the chat_id and send it using it.
Oh, and if I specify not my username, but Vasya?)