/**
 * 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;
 
		//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>\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";
    }
  }
 
}