在看php-openid的源码的时候，发现它实现的创建多级目录的方法的思想很巧妙，特将起代码记录下来：
/** * Create dir_name as a directory if it does not exist. If it * exists, make sure that it is, in fact, a directory. Returns * true if the operation succeeded; false if not. * * @access private */ static function ensureDir($dir_name) { if (is_dir($dir_name) || @mkdir($dir_name)) { return true; } else { $parent_dir = dirname($dir_name); // Terminal case; there is no parent directory to create. if ($parent_dir == $dir_name) { return true; } return (Auth_OpenID::ensureDir($parent_dir) && @mkdir($dir_name)); } }