How to write code correctly? Organize the development process?

Question

JavaScript

Hello. I am now writing my bikes using the "What would be interesting to do?" method. --> trying to do. And during the development process, many questions arise about its validity and readability.
For example:
1) I want to pass values ​​from PHP variables to JS, for this I echowrite a method in a var a = <? echo $var; ?>.
2) There are two JS scripts that should be executed as soon as the DOM is loaded. But I can’t combine it in one file, because there will be a visibility problem.
And similar problems. Lots of confusion in php and html. It all looks unreadable and wrong.
Question : how to organize the development process? What tools to use? What to read about this?
Do I need to use PHP in conjunction with JS? Because sometimes you have to work with the database. What technologies should be mastered?
I'm practicing. I want to become a confident Junior. Direction - JavaScript.

Answer 1

Stages of software development
1. Study of a given process or phenomenon
2. Development of terms of reference
3. Development of a mathematical model according to the received TOR
4. Selection of the programming language most suitable for the implementation of the received MM
5. Recording of the MM in the selected programming language
6. Debugging of the received
PS program
p. 1, 2, in theory, should be done by the customer, but this is only in theory.
It is better to divide a large problem into several smaller ones and solve each one separately. Perhaps even on different languages, in your case, part in PHP, part in JS? part in SQL

Answer 2

Google is full of information on data exchange between php and js, AJAX for example. To work with the database, you can use any programming language on the server side.