What is the order in which to develop and test a web application?

Question

PHP

Hello.
I'm trying to develop an application that is architecturally similar to the social. network - also users, also post content. There is no money to pay for the full development, so I want to try to develop the backend on my own, and when it is ready, order the development of mobile clients (the site is less priority). The technology stack chosen is as follows: NGINX, PHP, PostgreSQL. Actually, questions:
1. Should I somehow take into account when developing the server part the fact that clients will be mobile applications and not a regular site or a server, is it a server in Africa?
2. At the moment, the database has been designed and placed on a separate server. What should be the next step - testing the database (does the data move correctly, what load can it withstand, etc.) or write a backend and test it through requests to the backend (and from the backend to the database)? Or maybe it was even worth starting with installing NGINX?)
3. I know that in large projects there is no way without caching. Question: is it worth it to initially include caching in the project (in case the product becomes popular and there will be a lot of data) or can it be easily attached as needed?
Thanks in advance.

Answer 1

IMHO
1 Yes, it would be logical. As an option to conduct parallel development of API for mobile phones.
2 It is necessary to test the project as a whole, and each individual one. So you can test the database, but you can \ need to start developing the backend.
3 Caching - immediately. Then you will "rewrite again" for the cache to work.

Answer 2

The most important thing in such projects IMHO is safety. After defining the technology stack, I would start implementing user and permission management since this is the basis - a skeleton, if possible. Then the logic of the application itself and the access rights to view are already between users (My entries can be seen ... who?)