从dir获取信息时遇到麻烦 - （php）

This is my code:

$flags=FileSystemIterator::CURRENT_AS_FILEINFO | FileSystemIterator::SKIP_DOTS;
$ul = new FileSystemIterator(__DIR__."/../",$flags);
// $ul = new FileSystemIterator("/../",$flags);
foreach($ul as $file){
    if($file->isDir()){
        print $file->getFilename()."<br />";
    }
}

Its DIR is /var/www/html/dir1/dir2/dir3/test.php

It shows as the result of command : ls /var/www/html/dir1/dir2/

replace $ul = new FileSystemIterator(__DIR__."/../",$flags); with $ul = new FileSystemIterator("/../",$flags);

It shows as the result of command : ls /var/www/html/dir1/ Why the second show happen?