wp_nav_menu() 呼叫walk_nav_menu_tree() 这就叫Walker_Nav_Menu class.

在这些函数中有几个过滤器可以连接，但我想看看nav_menu_item_title 中的筛选器Walker_Nav_Menu 班以下是来源：

/**
* Filter a menu item\'s title.
*
* @since 4.4.0
*
* @param string $title The menu item\'s title.
* @param object $item  The current menu item.
* @param array  $args  An array of {@see wp_nav_menu()} arguments.
* @param int    $depth Depth of menu item. Used for padding.
*/
$title = apply_filters( \'nav_menu_item_title\', $title, $item, $args, $depth );

function my_nav_menu_item_title( $title, $item, $args, $depth ) {
    $title .= \'<span class="excerpt">\' . $item->post_excerpt . \'</span>\';
    return $title;
}
add_filter( \'nav_menu_item_title\', \'my_nav_menu_item_title\', 10, 4 );

因kingkool68解决

function my_nav_menu_item_title( $title, $item, $args, $depth ) {
  $pid = $item->object_id;

  $text = get_the_excerpt($pid);

  $title .= \'<span class="excerpt">\' . $text . \'</span>\';
  return $title;

}