PHP Memcache potential problems?

How could I guarantee that when the cache expires it'll get regenerated once and the data will be intact?

The approach to caching I take is, for lack of a better word, a "lazy" implementation. That is, you don't cache something until you retrieve it once, with the hope that someone will need it again. Here's the pseudo code of what that algorithm would look like:

// returns false if there is no value or the value is expired
result = cache_check(key)

if (!result)
{
    result = fetch_from_db()

    // set it for next time, until it expires anyway
    cache_set(key, result, expiry)
}

This works pretty well for what we want to use it for, as long as you use the cache intelligently and understand that not all information is the same. For example, in a hypothetical user comment system, you don't need an expiry time because you can simply invalidate the cache whenever a new user posts a comment on an article, so the next time comments are loaded, they're recached. Some information however (weather data comes to mind) should get a manual expiry time since you're not relying on user input to update your data.

For what its worth, memcache works well in a clustered environment and you should find that setting something like that up isn't hard to do, so this should scale pretty easily to whatever you need it to be.