Wordpress add page under admin submenu and retaining the active status of the parent submenu page in the menu

Question

I have added a page in the admin menu(pxmag-menu) and a submenu(pxmag-plans). There is another page(pxmag-plans-edit) set under the submenu(pxmag-plans) as the parent page.

public function __construct()
{
    require('pxMagAdminPlans.class.php');
    $this->admPlanObj= new pxMagAdminPlans();

    add_action('admin_menu', array($this, 'add_plan_admin_menu'));
}

public function add_plan_admin_menu()
{
    add_menu_page(__('Dashboard', 'textdomain'), get_bloginfo('name'), 'manage_options', 'pxmag-menu', array($this, 'pxmag_dash'), 'dashicons-welcome-view-site', 6);
        
    add_submenu_page('pxmag-menu', __('Subscription Plans', 'textdomain'), 'Plans', 'manage_options', 'pxmag-plans', array($this->admPlanObj, 'plan_admin_menu_page'));
    add_submenu_page('pxmag-plans', __('Add/edit Plans', 'textdomain'), 'Add/edit plans', 'manage_options', 'pxmag-plans-edit', array($this->admPlanObj, 'plan_admin_menu_edit'));
}

All the menu and submenu pages load fine.

But, when I open this page(pxmag-plans-edit), the menu selection in the Wordpress admin shows nothing as current item, whereas the pxmag-plans is supposed to be the current selection.

(It is supposed to work like: when I click 'Posts > Categories' and subsequently open the 'edit category' page, the 'Posts > Categories' option in menu keeps selected).

What is going wrong? What is the correct process?

Answer 1

If I understand it correctly — you want the "Plans" sub-menu to be highlighted when on the "Add/edit plans" (pxmag-plans-edit) page, then you can do it like so:

1. Use the add_menu_classes hook to highlight the pxmag-menu menu:

   function my_add_menu_classes( $menu ) {
       // Do nothing if not on the "Add/edit plans" page.
       global $plugin_page;
       if ( 'pxmag-plans-edit' !== $plugin_page ) {
           return $menu;
       }
   
       foreach ( $menu as $i => $item ) {
           if ( 'pxmag-menu' === $item[2] ) {
               $menu[ $i ][4] = add_cssclass( 'wp-has-current-submenu wp-menu-open', $item[4] );
           }
       }
   
       return $menu;
   }
   add_filter( 'add_menu_classes', 'my_add_menu_classes' );
   

2. Use the submenu_file hook to highlight the "Plans" (pxmag-plans) sub-menu:

   function my_submenu_file( $submenu_file, $parent_file ) {
       global $plugin_page;
       return ( 'pxmag-plans-edit' === $plugin_page )
           ? 'pxmag-plans' : $submenu_file;
   }
   add_filter( 'submenu_file', 'my_submenu_file', 10, 2 );
   

Answer 2

I have slightly modified the code posted as answer by @Sally CJ to work with multiple cases:

public function __construct()
{
    require('pxMagAdminPlans.class.php');
    require('pxMagAdminPromo.class.php');
        
    $this->admPlanObj= new pxMagAdminPlans();
    $this->admPromoObj= new pxMagAdminPromo();
        
    add_action('admin_menu', array($this, 'add_plan_admin_menu'));
        
    add_filter('add_menu_classes', array($this, 'adjust_menu_classes'));
    add_filter('submenu_file', array($this, 'adjust_submenu_file'), 10, 2 );
}

public function add_plan_admin_menu()
{
    add_menu_page(__('Dashboard', 'textdomain'), get_bloginfo('name'), 'manage_options', 'pxmag-menu', array($this, 'pxmag_dash'), 'dashicons-welcome-view-site', 6);
        
    add_submenu_page('pxmag-menu', __('Subscription Plans', 'textdomain'), 'Plans', 'manage_options', 'pxmag-plans', array($this->admPlanObj, 'plan_admin_menu_page'));
    add_submenu_page('pxmag-plans', __('Add/edit Plans', 'textdomain'), 'Add/edit plans', 'manage_options', 'pxmag-plans-edit', array($this->admPlanObj, 'plan_admin_menu_edit'));
        
    add_submenu_page('pxmag-menu', __('Manage Promotions', 'textdomain'), 'Promotions', 'manage_options', 'pxmag-promotions', array($this->admPromoObj, 'get_promotions'));
    add_submenu_page('pxmag-promotions', __('Add/Edit', 'textdomain'), 'Add/Edit', 'manage_options', 'pxmag-promotions-edit', array($this->admPromoObj, 'manage_promotions'));
        
    remove_submenu_page('pxmag-menu','pxmag-menu');
}
    
function adjust_submenu_file($submenu_file, $parent_file) {
    global $plugin_page;
    $retsub = $submenu_file;
    if('pxmag-plans-edit' === $plugin_page)
        $retsub = 'pxmag-plans';
    elseif ('pxmag-promotions-edit' === $plugin_page)
        $retsub = 'pxmag-promotions';
    return $retsub;
}

function adjust_menu_classes($menu)
{
    global $plugin_page;
    $retmenu = $menu;

    if (('pxmag-promotions-edit' == $plugin_page) || ('pxmag-plans-edit' == $plugin_page))
    {
        foreach ($menu as $i => $item)
        {
            if ('pxmag-menu' === $item[2])
            {
                    $retmenu[$i][4] = add_cssclass('wp-has-current-submenu wp-menu-open', $item[4]);
            }
        }
    }
    return $retmenu;
}