Add item to admin panel - w/o plugin & theme

Question

I'd like to create a new page in the wp-admin panel. I know I should use the add_menu_page() function for that and I understand how it works.

I'm new to WP development, and would just like to know in which file I should put this function. The doc does not answer to my question, or I don't understand it.

I don't know if it's possible but I'd like this page to be part of neither the plugins nor the themes.

Thanks for your help

Answer 1

As long as it gets registered before the admin_menu hook happens, it doesn't matter what file you put it in. The important part is what hook you call your function on. For simplicity sake you could put it in a themes functions.php. If you want things to look a bit cleaner then put it in a php file named admin-menu.php (or whatever explanatory name you want) and require_once that file from functions.php

add_action('admin_menu', 'your_function_name');
function your_function_name() {
    add_menu_page(...)// add your menu page here

}