Cacher is the code snippet organizer for pro developers

/**
 * Template tag to display subnavi, which is a sidebar navigation.
 *
 * It is to be used in hierarchical Page
 * Currently, it can only work when there are only one lv.3 exists.
 *
 * Usage example: echo tujstarter_subnavi( '14' );
 *
 * @since TUJ Starter 1.1
 * @todo To make it work when there are more than one lv.3 Pages. Smarter way to specify lv.3 page id.
 */
if ( ! function_exists( 'tujstarter_subnavi' ) ) :
function tujstarter_subnavi( $lv3root = 0 ) {
  // Leave a message and exit, if it is called when not in Page.
  if ( !is_page() ) {
    echo "<b>tujstarter_subnavi() is only usable when is_page() is true</b><br /><br />";
    return;
  } else {

    global $post;

    $subnavi_output ='';
    $root_url = '';
    $root_title = '';
    $args = array();

    // sanitize, mostly to keep spaces out
    $lv3root_id = preg_replace('/[^0-9,]/', '', $lv3root);

    // if this post/page has a parent (= lv.2 or 3 page), do this
    if( $post->post_parent ) {

      // get the parent's ID of this post/page.
      $subnavi_get_parent_id = get_post( $post->post_parent );
      // then use this parent's ID to get its parent's ID  and call this root
      $subnavi_root_page_id = $subnavi_get_parent_id->post_parent;

      // if root page id is zero it means lv.2, so get its parent's details
      if ( $subnavi_root_page_id == 0 ) {
        $root_url = get_permalink( $post->post_parent );
        $root_title = get_the_title( $post->post_parent );
        if ( $lv3root_id == $post->ID ) {
          // if this is a lv.3 Page specified in parameter, display all depth
          $args = array( 'title_li' => '', 'child_of' => $post->post_parent, 'echo' => 0, );
        } else {
          // if it is not, then display only 1 depth
          $args = array( 'depth' => 1, 'title_li' => '', 'child_of' => $post->post_parent, 'echo' => 0, );
        }
      } else {
        // otherwise, it means lv.3, so get its parent's parent's deitals
        $root_url = get_permalink( $subnavi_root_page_id );
        $root_title = get_the_title( $subnavi_root_page_id );
        $args = array( 'title_li' => '', 'child_of' => $subnavi_root_page_id, 'echo' => 0, );
      }

    } else {
      // otherwise, it means this post/page has no parent (=lv.1), so just get its own details
      $root_url = get_permalink( $post->ID );
      $root_title = get_the_title( $post->ID );
      $args = array( 'depth' => 1, 'title_li' => '', 'child_of' => $post->ID, 'echo' => 0, );
    }

    // lets construct and output if has any child pages to list
    $output_page_list = wp_list_pages( $args );
    
    if ( $output_page_list ) {
      // construct the title and list output in coding style of TUJ Core v4.
      $subnavi_output = '<nav class="secnavi-vert bm-30px">';
      $subnavi_output .= "\n\t<div>\n\t";
      $subnavi_output .= '<h1><a href="' . $root_url . '">' . $root_title . '</a></h1>';
      $subnavi_output .= "\n\t\t<ul>\n\t\t\t" . wp_list_pages( $args ) . "\n\t\t</ul>\t";
      $subnavi_output .= "</div>\n</nav>";

      // at last, return the output!
      return $subnavi_output;
    } else {
      return;
    } // END if ( $output_page_list )
  } // END if( $post->post_parent )
}
endif;