How to organize a function override in a child theme?

Question

WordPress

In the parent theme, the function is connected through the file at the end of functions.php.
In a child theme, I'm trying to override this function via

if (!function_exists('this_function')) {
    function this_function() {
        //  Ваш код.
    }
}

Due to connection rules I get:
Fatal error: Cannot redeclare this_function() (previously declared in ...\child-theme\functions.php:8) in ...\parent-themes\...\extras.php on line 163
Tell me how to change it.

Answer 1

The parent theme should have:

if (!function_exists('this_function')) {
    function this_function() {
        //  Ваш код.
    }
}

In the child - already without function_exists:

function this_function() {
   //  Ваш код.
}