How to properly separate what should be on the client side and on the server side?

Question

PHP

Good day, there are some rules that you need to follow. What should javascript ajax be responsible for what php. It is possible to write in php or javascript, for example, form validation, but if js is turned off in the browser, is it better to write this thing in php?
I know html, css, a bit of php js basics, but I didn’t dig deeper, I didn’t do tabs. Can you tell me how to tie it together?

Answer 1

> It is possible to write in php or javascript, for example, form validation, but if js is turned off in the browser, is it better to write in php?
It's good practice to do validation both on the front and on the back. For example, if the form is not valid and there is a validator on the front, then it will not be sent (saving time, resources, the user will receive an answer instantly). But in any case, it is impossible without validation on the back.