PHP class instantiation. To use or not to use the parentheses?

I've always assumed that - in the absence of constructor parameters - the parentheses (curly brackets) follow the class name when creating a class instance, were optional, and that you could include or exclude them at your own personal whim.

That these two statements were equal:

$foo = new bar;
$foo = new bar();

Am I right? Or is there some significance to the brackets that I am unaware of?

I know this sounds like a RTM question, but I've been searching for a while (including the entire PHP OOP section) and I can't seem to find a straight answer.

They are equivalent. If you are not coding by any code convention, use which you like better. Personally, I like to leave it out, as it is really just clutter to me.

$foo = new bar() would be useful over $foo = new bar if you were passing arguments to the constructor. For example:

class bar {

    public $user_id;

    function __construct( $user_id ) {
        $this->user_id = $user_id
    }
}

-

$foo = new bar( $user_id );

Aside from that, and as already mentioned in the accepted answer, there is no difference.