Custom Simple Section Navigation for posts that don't have a parent

custom-ssn.php

<?php

add_shortcode( 'ssn', 'esw_custom_ssn' );
/**
 * Emulate Simple Section Nav for a specific parent post/page.
 *
 * Example: [ssn ancestor="ancestor_id" parent="parent_id" depth="depth"]
 *
 * @since   1.0.0
 *
 * @param   array  $atts  Shortcode atts.
 *
 * @return  string  Output HTML.
 */
function esw_custom_ssn( $atts ) {
    
    global $post;

	if ( ! isset( $atts['parent'] ) ) {
		$atts['parent'] = '';
	}

    /**
     * Global variable to hold shortcode parent value
     *
     * Fix this properly with OOP practice: 
     * http://wordpress.stackexchange.com/questions/45901/passing-a-parameter-to-filter-and-action-functions
     */
    global $esw_ssn_parent_ids;
    $esw_ssn_parent_ids[] = $atts['parent'];

    /** Get shortcode parameters */
    extract( shortcode_atts( array(
        'ancestor' => '',
        'parent' => '',
        'depth' => '1',
        'show_all' => false, 
        'exclude' => '', 
        'hide_on_excluded' => true, 
        'show_on_home' => false, 
        'show_empty' => false, 
        'sort_by' => 'menu_order', 
        'a_heading' => false, 
        'before_widget' => '<div class="custom-ssn simple-section-nav" data-parent="' . $atts['parent'] . '">',
        'after_widget'=>'</div>', 
        'before_title'=>'<h4 class="widget-title">', 
        'after_title'=>'</h4>', 
    ), $atts ) );

    // Add class for current parent
    add_filter( 'page_css_class', 'esw_apply_custom_ssn_classes', 10, 6 );

    // Only if $ancestor is set 
    if ( ! empty( $ancestor ) ) {

        $content = $before_widget . $before_title . get_the_title( $ancestor ) . $after_title . '<ul>';

        $content .= wp_list_pages( array( 
            'child_of' => $ancestor,
            'title_li' => '',
            'depth' => $depth,
            'sort_column' => $sort_by,
            'exclude' => $exclude,
            'echo' => false,
            )
        );

        $content .= '</ul>' . $after_widget;

        return $content;
    
    }

}


/**
 * Add 'current' class to 
 *
 * @since   1.0.0
 *
 * @see     wp_list_pages()
 *
 * @param   array    $css_class     An array of CSS classes to be applied
 *                                to each list item.
 * @param   WP_Post  $page          Page data object.
 * @param   int      $depth         Depth of page, used for padding.
 * @param   array    $args          An array of arguments.
 * @param   int      $current_page  ID of the current page.
 *
 * @return  string  			   Updated CSS classes.
 */
function esw_apply_custom_ssn_classes( $css_class, $page, $depth, $args, $current_page ) {
	
	global $esw_ssn_parent_ids;

	// Get all ancestors
	$ancestors = get_post_ancestors( $page->ID );
	
	/**
	 * Add 'current' class if:
	 * - Page ID is in parent array
	 * - Page ancestor array matches ancestor array
     */
	if ( in_array( $page->ID, $esw_ssn_parent_ids ) ||
		 array_intersect( $ancestors, $esw_ssn_parent_ids )

	) {
		$css_class[] = 'current_page_parent';
	}
			
	return $css_class;
}

?>