I am planning to create a broadcasting channel app.
there is a group of people who are managing 20-25 whatsapp groups and are sending religious messages.
They tried broadcasting feature of whatsapp but they prefer managing group which makes sure that people are receiving messages.
broadcaster's number needs to be saved in receivers contacts for whatsapp broadcasting and all subscribers were not doing so, and complaining about not receiving messages.

so this group decided to manages whatsapp groups and put guidelines that no users other than admins are allowed to post anything.

so ultimately they want a broadcasting medium.

another problem here is that it is hard to find old messages to refer to. they are posting everything on blog too, but not all the subscribers are comfortable using that.

so I am planning to create an app which will store the received messages offline(last 30 days) and anyone can read older messages within the app and also can share it on other social media.

would you guys please suggest me architecture for this app?
I have learned PHP and thinking about using phalcon PHP framework, but it required VPS and it is costly.

any suggestions welcomed.