test-tabs.php

<?php
$folder = ['file1.txt', 'file2.txt', 'subfolder1' => ['subfolder2' => ['file3']]] ;
$printFolder = function( $folder, $key = 'folder', $i = 0 ) use ( &$printFolder )
{
	$tab = str_repeat ('--', $i);
	if ( isset ( $key ) ) 
		print ( "$tab $key:\n" );
	
	foreach ($folder as $key => $file) {
		if ( is_array ( $file ) ) {
			$i++;
			if ( isset ( $key ) )
				$printFolder ( $file, $key, $i );
			else 
				$printFolder ( $file, null, $i );
		} else {
			$tab .= '--';
			print ( "$tab $file\n" );		
		}
	}
};

$printFolder ( $folder );