If a variable from a class is false in php

I'm a novice php programmer and I've been looking for an answer for this to no avail. Basically, let's say I have a class (let's call it server) and I have a variable in that class (let's call it $enabled). If I want to find what happens when it's false, could I do this:

if (server::!$enabled) {}

or would I have to do something like this:

if (!(server::$enabled)) {}

or something else entirely?