$flags stays untouched if $key was not found on the server, it's helpfull to determine if bool(false) was stored:
<?php
$memcache = new Memcache();
$memcache->set('test', false); //
$flags = false;
var_dump($memcache->get('test', $flags)); // bool(false)
var_dump($flags); // int(256) - changed to int
$memcache->delete('test');
$flags = false;
var_dump($memcache->get('test', $flags)); // bool(false)
var_dump($flags); // bool(false) - untouched
?>