function deletekeysbyindex($prefix = false) {
    $m = new Memcached();
    $m->addServer('localhost', 11211);
    $keys = $m->getAllKeys();
    if ($prefix !== false) {
        foreach ($keys as $index => $key) {
            if (strpos($key,$prefix) !== 0) {
                unset($keys[$index]);
            } else {
                $m->delete($key);
            }
        }
    }
    return $keys;
}