How to make a condition when a certain id appears on the page?

Question

PHP

How can I track whether the given id="###" appeared on the page or not?
Let's say I have blocks:
blah blah
blah
blah blah
They are dynamic, each appears in certain conditions.
I need that if id="1" appears, id="2" disappears
Roughly speaking:
if (id="3"-present)
{
take id="2" and add ' style="display"none" to it ' (or new class)
}
Please help me to do this in php.

Answer 1

isset() , but it's better to take js for this

Answer 2

How to trigger an event in javascript

Answer 3

I do not know if you use third-party libraries in the project. But if that appear.js verstaemvse.ru/scroll-plugin-appear-js.html it determines whether there is an element in view when scrolling. And I don’t understand something, should they appear and disappear when scrolling, or what?