How to develop a web app taking into account further expansion and loads?

Question

PHP

Hello. An amusing idea came to my mind, I decided to collect information, read it and remember what and what.
Technology stack: php/MySQL and here is the first question: is it better to take a framework/microframework/write a microframework yourself?
The second question: how to carry out registration (namely the technical part) in order to be convenient and safe (where to read about the correct use of registration through social networks and the best practice implementation mechanism)
How to avoid panic in the future? What should be taken into account when developing, so that later you do not pull out your hair everywhere?
Is it worth making a mobile version, or is it better just an application?
Should I use bootstrap || foundation, if it's worth it, which is better to use?
Thanks in advance. I will be glad to self story, links to information and a clear presentation.