<?php

$adapter = new Gaufrette\Adapter\Ftp(/* ... */);

foreach ($adapter->keys() as $key) {
    if (!$adapter->isDirectory($key)) {
        $adapter->delete($key);
    }
}
$keys = $adapter->keys();
rsort($keys);
foreach ($keys as $key) {
    $adapter->delete($key);
}

return $adapter;