goto next folder on same level

Example:

ROOT/Folder1/index.php
ROOT/Folder2/index.php
ROOT/Folder3/index.php (Current Folder)

I would like to insert code in each index.php file that would allow the user to goto previous / next folders on the same level.

So far I have this much code that displays all files in the current folder but how do you ../ to the parent folder and display only the folders?

<?php
if ($handle = opendir('.')) {
while (false !== ($file = readdir($handle))) {
if ($file != "." && $file != "..") {
echo "$filen";
?><br><?
}
}
closedir($handle);
}
?>

to display only folders, do an if(is_dir($file))