I had some issues recently where I was trying to edit a client website to have custom options page and sub options pages using the Wordpress plugin Advanced Custom Fields (ACF). According to their documentation, it's very simple to do.

But I ran into 2 silly problems that I didn't account for. If you're not able to get the provided code to work, here are the 2 reasons why it didn't work for me:

  1. You have to have the Pro version of the plugin
  2. There can only be 1 options page - the theme already had one that I didn't know about

If you're not sure if your website already has an options page, do a search all files on the function 'acf_add_options_page'

Otherwise, here is a code example to get this working with a main options page and a sub page:

if( function_exists('acf_add_options_page') ) {

    $parent = acf_add_options_page(array(
        'page_title'  => 'Theme General Settings',
        'menu_title'  => 'Theme Settings',
        'menu_slug'   => 'theme-general-settings',
        'capability'  => 'edit_posts',
        'redirect'    => false
    ));

    acf_add_options_sub_page(array(
        'page_title'  => 'Subpage',
        'menu_title'  => 'Subpage',
        'parent_slug' => $parent['menu_slug'],
    ));
}

In the code above, acf_add_options_sub_page() function is used to create a subpage. This function takes an array of arguments similar to acf_add_options_page(). The key difference is that it includes a 'parent_slug' parameter. The 'parent_slug' parameter refers to the slug of the parent options page.