How to access variables in the function where apply_filters() is called?

Question

I am trying to filter some data based on whether a variable from within the function that calls apply_filters() is equal to specific value. That variable is not passed to the apply_filters() parameters. This might explain what I mean:

// function in wordpress core
function get_var_b() {
    // generating $var_a
    $var_a = "a";
    // some code ...
    $var_b = apply_filters('get_var_b', $var_b);
    return $bar_b;
}

// in functions.php
add_filter('get_var_b', 'filter_var_b');
function filter_var_b($var_b) {
    if ($var_a == "c") {
        $var_b = "d";
    }
    return $var_b;
}

I tried to use global $var_a; inside of my function but with no success. The only way I could access $var_a was by editing the core file and passing $var_a to the apply_filters() function like:

apply_filters('get_var_b', $var_b, $var_a);

Is there a way to access this variable without editing the core file or trying to regenerate $var_a from scratch? Thanks.

EDIT: The filter that I am trying to hook to is dynamic_sidebar_params which is called from dynamic_sidebar() function in wp-includes/widgets.php file. I am trying to change the $params[0]['before_widget'] from <aside> to <nav> IF the $id was equal to categories-2.

Answer 1

Short answer: you can't.

You're trying to access a variable outside of its scope. In your situation, $var_a is local to the get_var_b() function, so anything outside of that function cannot see it.

This is equivalent to writing this sequence in PHP:

function get_var_b() {
    // This variable is local to the function and invisible outside of it.
    $var_a = "a";  

    // Call a function that returns some value while passing in some default value.
    $var_b = create_var_b( 'default' ); 

    return $var_b;
}

function create_var_b( $default = null ) {
    // Inside this function, we only have access to $default because that's all that was passed
    // in. You can access any set global variables my making an explicit reference to 
    // "global $var", but only if the variable was globalized in its original scope.

    if ($default != null) {
        $var_b = "d";
    } else {
        $var_b = "c";
    }

    return $var_b;
}