How to obtain Telegram chat_id for a specific user?

Question

How to obtain user chat_id in Telegram bot API? The documentation says:

Integer | Unique identifier for the message recipient — User or G

Answer 1

Straight out from the documentation:

Suppose the website example.com would like to send notifications to its users via a Telegram bot. Here's what they could do to enable notifications for a user with the ID 123.

1. Create a bot with a suitable username, e.g. @ExampleComBot
2. Set up a webhook for incoming messages
3. Generate a random string of a sufficient length, e.g. $memcache_key = "vCH1vGWJxfSeofSAs0K5PA"
4. Put the value 123 with the key $memcache_key into Memcache for 3600 seconds (one hour)
5. Show our user the button https://telegram.me/ExampleComBot?start=vCH1vGWJxfSeofSAs0K5PA
6. Configure the webhook processor to query Memcached with the parameter that is passed in incoming messages beginning with /start. If the key exists, record the chat_id passed to the webhook as telegram_chat_id for the user 123. Remove the key from Memcache.
7. Now when we want to send a notification to the user 123, check if they have the field telegram_chat_id. If yes, use the sendMessage method in the Bot API to send them a message in Telegram.