Examples of extending WordPress Walker_Nav_Menu for sibling and subpage menus

functions.php

<?php
/**
 * Custom Walker to extract current sister-sub-menu
 */

class Custom_Walker_Nav_Sister_Menu extends Walker_Nav_Menu {

var $found_parents = array();
function start_el(&$output, $item, $depth, $args) {

		global $wp_query;
		
	
		//this only works for second level sub navigations
		$parent_item_id = 0;

		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

		$class_names = $value = '';

		$classes = empty( $item->classes ) ? array() : (array) $item->classes;	

		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
		$class_names = ' class="' . esc_attr( $class_names ) . '"';
		
		
	 if ( ($item->menu_item_parent==0) && (strpos($class_names, 'current-menu-parent')) ) { 
$output.= '
<li>';
		 }


		// Checks if the current element is in the current selection
		if (strpos($class_names, 'current-menu-item')
			|| strpos($class_names, 'current-menu-parent')
			|| strpos($class_names, 'current-menu-ancestor')
			|| (is_array($this->found_parents) && in_array( $item->menu_item_parent, $this->found_parents )) ) {

			// Keep track of all selected parents
			$this->found_parents[] = $item->ID;

			//check if the item_parent matches the current item_parent
			if($item->menu_item_parent!=$parent_item_id){

				$output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

				$attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
				$attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
				$attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
				$attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

				$item_output = $args->before;
				$item_output .= '<a'. $attributes .'>';
				$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
				$item_output .= '</a>';
				$item_output .= $args->after;
				
				$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
			}
			
			
		}
	}
	
	function end_el(&$output, $item, $depth) {
	  // Closes only the opened li
	  if ( is_array($this->found_parents) && in_array( $item->ID, $this->found_parents ) ) {
	      $output .= "</li><!--". $item->menu_item_parent."-->";
    }
  }

  function end_lvl(&$output, $depth) {
    $indent = str_repeat("\t", $depth);
    // If the sub-menu is empty, strip the opening tag, else closes it
    if (substr($output, -22)=="<ul class=\"sub-menu\">\n") {
      $output = substr($output, 0, strlen($output)-23);
    } else {
      $output .= "</ul>";
    }
	  $emptysubmenu = '<ul class="sub-menu">




</ul>';

  $output = str_replace($emptysubmenu, '', $output);
  }  
}

/* END SISTER CUSTOM MENU WALKER */



/**
 * Custom Walker to extract current sub-menu
 */

class Custom_Walker_Nav_Sub_Menu extends Walker_Nav_Menu {

var $found_parents = array();
function start_el(&$output, $item, $depth, $args) {

		global $wp_query;
		$id = get_the_ID();
		
	
		//this only works for second level sub navigations
		$parent_item_id = 0;

		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

		$class_names = $value = '';

		$classes = empty( $item->classes ) ? array() : (array) $item->classes;	

		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
		$class_names = ' class="' . esc_attr( $class_names ) . '"';
		
		
	 if ( ($item->menu_item_parent==0) && (strpos($class_names, 'current-menu-parent')) ) { 
		 $output.= '
<li>';
		 }


		// Checks if the current element is in the current selection
		
		if (strpos($class_names, 'current-menu-item')
			|| (is_array($this->found_parents) && in_array( $item->menu_item_parent, $this->found_parents )) ) {
			
			
			// if ($item->menu_item_parent==$id) {

			// Keep track of all selected parents
			$this->found_parents[] = $item->ID;

			//check if the item_parent matches the current item_parent
			if($item->menu_item_parent!=$parent_item_id  && !strpos($class_names, 'current-menu-item') ){

				$output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

				$attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
				$attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
				
				if (!strpos($class_names, 'current-menu-item')) {
					$attributes .= ' id="subpage_link_'    . $item->object_id .'"';
					$attributes .= ' class="show-subpage-ajax"';
				}
				
				$attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

				$item_output = $args->before;
				$item_output .= '<a'. $attributes .'>';
				$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
				$item_output .= '</a>';
				$item_output .= $args->after;
				
				$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
			}
			
			
		}
	}
	
	function end_el(&$output, $item, $depth) {
	  // Closes only the opened li
	  if ( is_array($this->found_parents) && in_array( $item->ID, $this->found_parents ) ) {
	      $output .= "</li>\n";
    }
  }

  function end_lvl(&$output, $depth) {
    $indent = str_repeat("\t", $depth);
    // If the sub-menu is empty, strip the opening tag, else closes it
    if (substr($output, -22)=="<ul class=\"sub-menu\">\n") {
      $output = substr($output, 0, strlen($output)-23);
    } else {
      $output .= "$indent</ul>\n";
    }
	  $emptysubmenu = '<ul class="sub-menu">

</ul>';
  $output = str_replace($emptysubmenu, '', $output);
  }  
}

/* END SUBPAGE CUSTOM MENU WALKER */