Why declare vars in a cool constructor place?

Is there any good reason to declare variables used by the class outside the constructor function first?

class foo { 

    var bar;  // why is this a good practice? (or could it be skipped?)

    public function __construct() {
        $this->foo = 'foobar';
    }
}

I have seen this often, but I'm not sure what he is doing, as it seems that he works great to leave them.