Extending ArrayObject in PHP properly?

Problem: I am trying to extend PHP's ArrayObject as shown below. Unfortunately I can't get it to work properly when setting multi-dimensional objects and instead an error thrown as I have the strict settings enabled in PHP. (Error: Strict standards: Creating default object from empty value)

Question: How can I modify my class to automatically create non-existing levels for me?

The code:

$config = new Config;
$config->lvl1_0 = true; // Works
$config->lvl1_1->lvl2 = true; // Throws error as "lvl1" isn't set already

class Config extends ArrayObject
{
    function __construct() {
        parent::__construct(array(), self::ARRAY_AS_PROPS);
    }

    public function offsetSet($k, $v) {
        $v = is_array($v) ? new self($v) : $v;
        return parent::offsetSet($k, $v);
    }
}