Here is my configure:

    $cache = new Phalcon\Cache\Backend\Memcache($frontCache, array(
        "prefix" => "cache",
        "host" => "localhost",
        "port" => "11211"
    ));

` Here is the code that function delete() not working:

    $keys = $this->cache->queryKeys();
    foreach($keys as $key) {
        $this->cache->delete($key)
    }

`

If prefix is empty "prefix" => "" it works properly