How do you use a @ symbol in the name of a PHP object

I've got an XML file that has the label @attributes for one of the names

SimpleXMLElement Object
(
    [@attributes] => Array
        (
            [PART_NUMBER] => ABC123

I want to make a reference to this object like $product->@attributes['part_number'] but of course the @ symbol causes an error.

So how do I reference this item in the object?