How to access superglobals in correct way?

As the hint says, accessing the superglobals violates the encapsulation principle

A really basic approach would be:

class SessionObject
{
    public $vars;

    public function __construct() {
        $this->vars = &$_SESSION; //this will still trigger a phpmd warning
    }
}

$session = new SessionObject();
$session->vars['value'] = "newValue";

You can also have a look to the Symfony HttpFoundation Component for a full-fledged implementation