Things are moving so fast in my head that I'm starting to edit... like a telegram.
This is a phpBB extension that sends a message to any Telegram chat (group chat or private chat) via a Telegram bot whenever someone makes a new topic or replies to an existing topic on the phpBB forum.
The code is based off of phpbb/phpbb-ext-acme-demo and haivala/phpBB-Entropy-Extension (a similar extension for Slack notifications).
Make your own Telegram bot.
Clone this Git repository into phpBB/ext/lassik/telegramnotifications:
$ cd path/to/your/phpBB
$ git clone https://github.com/lassik/phpbb-ext-telegramnotifications.git ext/lassik/telegramnotifications
In phpBB, go to "ACP" > "Customise" > "Extensions" and enable the "Telegram Notifications" extension.
Then to "ACP" > "Extensions" > "Telegram settings" and fill in the settings. You get the auth token when you make the bot. To get the chat ID, try the "Find chat ID" page or from the sidebar or see https://stackoverflow.com/questions/32423837/telegram-bot-how-to-get-a-group-chat-id.
Now try to make new posts and see if you get notified on Telegram!