Trying to list all folders in a directory with PHP, need help…

Heya,
I’m trying to list only the folders in this directory but as soon as I add the is_dir($file) it lists nothing. If I remove it it will list files and directories. Can someone help? Thanks!

if ($handle = opendir('../wp-content/plugins/tpc-slideshow/themes')) {

	while (false !== ($file = readdir($handle))) {
		if( $file != '.' && $file != '..' && is_dir($file) ) {
			echo "<p>$file</p>";
		}
	}

	closedir($handle);
}