json_decode to custom class

Not automatically. But you can do it the old fashioned route.

$data = json_decode($json, true);

$class = new Whatever();
foreach ($data as $key => $value) $class->{$key} = $value;

Or alternatively, you could make that more automatic:

class Whatever {
    public function set($data) {
        foreach ($data AS $key => $value) $this->{$key} = $value;
    }
}

$class = new Whatever();
$class->set($data);

Edit: getting a little fancier:

class JSONObject {
    public function __construct($json = false) {
        if ($json) $this->set(json_decode($json, true));
    }

    public function set($data) {
        foreach ($data AS $key => $value) {
            if (is_array($value)) {
                $sub = new JSONObject;
                $sub->set($value);
                $value = $sub;
            }
            $this->{$key} = $value;
        }
    }
}

// These next steps aren't necessary. I'm just prepping test data.
$data = array(
    "this" => "that",
    "what" => "who",
    "how" => "dy",
    "multi" => array(
        "more" => "stuff"
    )
);
$jsonString = json_encode($data);

// Here's the sweetness.
$class = new JSONObject($jsonString);
print_r($class);