beginner's MySQL/memcache/PHP question

[Josh Meyer]

I use mysqli in PHP 5 right now and want to start incorporating the use 
of memcache.  I'm able to store something in memcache, but I'm not quite 
able to get things out. 

I put data in with this:
    $db = new mysqli('host','user','password','database');
    $sql = ... <a SELECT statement>
    $result = $db->query($sql);
    $memcache->set($memcache_key,$result,TRUE,86400) or die ("Failed to 
save data at the server");

The output of this:
    $cached_result = $memcache->get($memcache_key);
    var_dump($cached_result);

is this:
    object(mysqli_result)#2 (0) { }

So, shouldn't I be able to treat $cached_result as a mysqli result set?  
Shouldn't something like the following work (it doesn't)?
    $num_results = $cached_result->num_rows;

Should I massage $result into something else before sticking it into 
$memcache->set()?

Thanks in advance,
Josh