I don't want to put down the effort you put into this, but using OO for this a task seems like overkill. How is

$hot->count("www.yahoo.com","200"); #set to 200
$hot->count("www.yahoo.com","+20"); #add 200
$hot->count("www.yahoo.com"); #increase by 1
[download]

any better than a hash?

$hot{"www.yahoo.com"} = 200;
$hot{"www.yahoo.com"} += 20;
$hot{"www.yahoo.com"}++
[download]

Note also how the latter makes it immediately obvious what's going on - you don't have to look up a classes' behaviour to know what it does.

Lastly, if I did take the OO route for some reason - maybe because I need to pass this around a lot of places in my program, and maybe it does a lot more on-the-fly calculation than just counting mentions, or something - I'd prefer an interface like this:

$hot->count("www.yahoo.com")->set(200);
$hot->count("www.yahoo.com")->add(20);
$hot->count("www.yahoo.com")->inc;
[download]

Makeshifts last the longest.