You have a missing class import. Try importing this class: League\Flysystem\Cached\Storage\Memcached
use League\Flysystem\Filesystem;
use League\Flysystem\Adapter\Local as Adapter;
use League\Flysystem\Cached\CachedAdapter;
use League\Flysystem\Cached\Storage\Memcached as MemcachedStore;
$memcached = new Memcached;
$memcached->addServer('localhost', 11211);
$adapter = new CachedAdapter(
new Adapter(__DIR__.'/path/to/root'),
new MemcachedStore($memcached, 'storageKey', 300)
);
$filesystem = new Filesystem($adapter);
// Storage Key and expire time are optional