Example:

$user = "test_user";
$allowed = 8;

$instance = new COM("dll.CObject") or die("Unable to
instanciate dll.CObject");
$instance->CheckLogin($user, COM_REMOTE_SERVER, &$allowed);

echo $allowed;

$allowed has always the value that you have specified
above, here: 8; When I change $allowed to 6788, then
I get 6788. It seems that the variable won't be changed
when you pass it by reference.
It doesn't matter if i use the & sign or not!
Note that the variable $allowed have to be integer,
I can't pass a variant type to the COM-Function CheckLogin.

The same has been working with previous versions of PHP,
until 4.0.5 and above.

In need this functionality because i use a lot of stuff
like that: 

$instance->some_function(&$var1, &$var2, &$var3);

any clue, what's going up?