How to access a function declared in child theme’s functions file in a plugin file?

you can create a hook to make communication between different parts :

in your theme :

add_filter("MY_THEME/creating_something", function ($return, $args1, $args2) {
    
    error_log("calling creating_something in " . __FILE__);
    
    if ($args2 > 2) {
        $return = 100 * $args1 + $args2;
    }
    
    return $return;
    
}, 10, 3); // 3 is for the 3 arguments of the anonymous function

and everywhere else you can use it like that :

$result = apply_filters("MY_THEME/creating_something", 45, 0, 0);
// return 45

$result = apply_filters("MY_THEME/creating_something", 45, 30, 9);
// return 3009

in my example, 10 is the common priority for hooks. you can use different priority if, e.g., you want to override this filter in you development environment, you can do that :

add_filter("MY_THEME/creating_something", function ($return, $args1, $args2) {
    
    return "It's dev here";
    
}, 500, 3);

tech