Consistent Hashing is supported by PHP's memcached extension. You don't have to do anything except make use of it in your code like this:
<?php
$servers = array(
array('memcache1.example.com', 11211),
array('memcache2.example.com', 11211)
);
$m = new Memcached();
$m->setOption(Memcached::OPT_DISTRIBUTION, Memcached::DISTRIBUTION_CONSISTENT);
$m->addServers($servers);
?>
When you then start adding items to the cache, the extension distributes them to the servers automatically so that it minimizes cache losses in case you add server(s). In case it cannot retrieve an item from a server where it's supposed to be - because the server is down for example - you will have to handle that in your php-code yourself.
Using addServers instead of addServer makes no difference for Consistent Hashing. As stated in the documentation you should use addServers though when adding multiple servers so that the internal datastructures are only updated once.
PHP's implementation of Consistent Hashing is based on libketama it does not need libketama at all. The extension takes care of distributing the items to the different servers for you so that it minimizes cache losses. There will always be an impact when servers are removed or added.