How to access an object property with a minus-sign?

I got an object (in PHP) and I can't print the content. In debug-mode it's like this:

stdClass Object
(
    [pre-selection] => 1
)

But I can't print the 'pre-selection' because of the minus sign.

echo $object->pre-selection; //doens't work.

How can I print this out? Thanks.