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

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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 );