How could I refer a variable outside a custom walker class in a custom walker

How could I refer a variable, which is outside a custom walker class(I think this is a loop), in a custom walker?

I have to use a variable(array) in the walker, but it should be declared out of the walker(because the class file is a loop) and should be able to refer from outside the walker:

$some_array = array();

class Custom_Walker_Nav_Menu extends Walker {
…
function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {

// I want to use the variable ‘$some_array‘ here

}
}