The right choice seems to be to turn on ketama-weighted by default...
ruby - fauna memcached (libmemcached wrapper) uses ketama-weighted
:ketama_weighted => true,
For PHP, the default distribution is MODULA distribution. However, from documentation in http://www.php.net/manual/en/memcached.constants.php, the safety tips around ketama hashing...
Enables or disables compatibility with libketama-like behavior. When enabled, the item key hashing algorithm is set to MD5 and distribution is set to be weighted consistent hashing distribution. This is useful because other libketama-based clients (Python, Ruby, etc.) with the same server configuration will be able to access the keys transparently.
It is highly recommended to enable this option if you want to use consistent hashing, and it may be enabled by default in future releases.