<?php
$m = new Memcached();
$m->addServer('localhost', 11211);

do {
    $ips = $m->get('ip_block', null, $cas);

    if ($m->getResultCode() == Memcached::RES_NOTFOUND) {

        $ips = array($_SERVER['REMOTE_ADDR']);
        $m->add('ip_block', $ips);

    } else {

        $ips[] = $_SERVER['REMOTE_ADDR'];

        $m->cas($cas, 'ip_block', $ips);
    }

} while ($m->getResultCode() != Memcached::RES_SUCCESS);