What are Ruby variables preceded with double at signs (@@)? My understanding of a variable preceded with an at sign is that it is an instance variable, like this in PHP:

PHP version

class Person {

    public $name;

    public function setName($name) {
        $this->name = $name;
    }

    public function getName() {
        return $this->name;
    }
}

Ruby equivalent

class Person

    def set_name(name)
        @name = name
    end

    def get_name()
        @name
    end
end

What does the double at sign @@ mean, and how does it differ from a single at sign?