When using setSaslAuthData(), it's a good change you'll get the following warning:
Warning: Memcached::setSaslAuthData(): SASL is only supported with binary protocol in test.php on line 8
To enable binary protocol you'll have to set the Memcached::OPT_BINARY_PROTOCOL option to true:
<?php
$mc->setOption(Memcached::OPT_BINARY_PROTOCOL, true);
?>