您声明了相同的函数名，这就是“声明的错误”的原因。只需更改类函数（包括在过滤器挂钩中）。或者您可以扩展您的函数代码。我还将重写if结构，使其更易于阅读：

add_filter( \'nav_menu_css_class\', \'add_custom_class\', 10, 2 );

function add_custom_class( $classes = array(), $menu_item = false ) {

    //Check if already have the class
    if (! in_array( \'current-menu-item\', $classes ) ) {

        //Check if it\'s the ID we\'re looking for
        if ( 1633 == $menu_item->ID ) {

            //Check if is in a single post
            if ( is_single() ) {

                //Check if the single post is in the category
                if ( in_category( \'Services\' ) ) {

                    $classes[] = \'current-menu-item\';

                }

            }

        } elseif ( 1634 == $menu_item->ID ) {

            //Check if is in a single post
            if ( is_single() ) {

                //Check if the single post is in the category
                if ( in_category( \'Products\' ) ) {

                    $classes[] = \'current-menu-item\';

                }

            }

        }

    }

    return $classes;
}

此外，此代码几乎是非动态的，因为必须指定菜单ID和类别名称。如果我理解了你的菜单逻辑，那么你是在试图将“当前菜单项”添加到一个类别菜单中，而一篇文章有这个类别。我会编写类似的代码：（因此它适用于任何类别和帖子组合）：

add_filter( \'nav_menu_css_class\', \'add_custom_class\', 10, 2 );

function add_custom_class( $classes = array(), $menu_item = false ) {

    //Check if already have the class
    if (! in_array( \'current-menu-item\', $classes ) ) {

        //Check if it\'s a category
        if ( \'category\' == $menu_item->object ) {

            //Check if the post is in the category
            if ( in_category( $menu_item->ID ) ) {

                $classes[] = \'current-menu-item\';

            }

        }

    }

    return $classes;
}