apply_filters( 'nav_menu_css_class', $classes, $item, $args, $depth )

function nav_menu_css_class_func ( $classes, $item, $args, $depth ) {
	$classes[] = 'nav-item'; // 添加class属性
	// 替换class属性
	$classes = preg_replace('/current-menu-item/i', 'active', $classes);
	$classes = preg_replace('/menu-item-has-children/i', 'dropdown', $classes);
	return $classes;
}
add_filter( 'nav_menu_css_class', 'nav_menu_css_class_func', 10, 4 );