The method correctly returns false if you set the value to false. This means that in order to have proper fault checking mechanism in place you need to check the result code.
<?php
$Memcached = new Memcached();
$Memcached->addServer('localhost', 11211);
$Memcached->set('key', false);
var_dump($Memcached->get('key')); // boolean false
var_dump($Memcached->getResultCode()); // int 0 which is Memcached::RES_SUCCESS
?>