dirname(__FILE__).DIRECTORY_SEPARATOR非常有用。。DIRECTORY_SEPARATOR代表\或者/
 
if(!defined('PHPCMS_PATH')) define('PHPCMS_PATH', PC_PATH.'..'.DIRECTORY_SEPARATOR);找上级目录

测试代码

//DIRECTORY_SEPARATOR 代表“\”
 define('PC_PATH',dirname(__FILE__).DIRECTORY_SEPARATOR);
 //代表找到当前目录的上一级目录等同于$_SERVER['DOCUMENT_ROOT'];
  define('PHPCMS_PATH', PC_PATH.'..'.DIRECTORY_SEPARATOR);
 $path = PHPCMS_PATH.'update.php';
 if(!file_exists($path))
 {
  echo 'hello';
 }
 else 
 {
  echo 'world';
 }