Not sure Extend walker in Functions.php to set up

class Mega_Menu_Walker extends Walker_Nav_Menu {
function start_el(&$output, $item, $depth=0, $args=[], $id=0) {
$output .= “

class Mega_Menu_Walker extends Walker_Nav_Menu {
function start_el(&$output, $item, $depth=0, $args=[], $id=0) {
        $output .= "<li class='" .  implode(" ", $item->classes) . "'>";
 
        if ($item->url && $item->url != '#') {
            $output .= '<a href="' . $item->url . '">';
        } else {
            $output .= '<span>';
        }
 
        $output .= $item->title;
 
        if($item->url && $item->url !='#'){   
            $output .= '</a>';
        }else{  
            $output .= '</span>';
        }
    }
}

}　　

functions.php i put

    // 新しいメニューを出力する
    wp_nav_menu( array(
        'menu'            => 'mega_menu', // メニュー管理画面で登録したメニュー名
        'container'       => '',
        'container_id'    => '',
        'container_class' => '',
        'menu_id'         => '',
        'menu_class'      => 'nav navbar-nav',
        'walker'          => new Mega_Menu_Walker
    ));
}


// ナビゲーションメニューのループ終了
?>

header.php i put

i have problem facing “extend walker” fucntion.php How Can i read function.php written above to header navigation to add or change.

help me tanks