在object方法中返回一个值还是在object方法中设置值? PHP

I can write classes which do the exact same thing two different ways;

class Simple {

    private $var;

    public function __construct() {

        $this->var = $this->setVar();
    }

    private function setVar() {

        return true;
    }
}

Or

 class Simple {

    private $var;

    public function __construct() {

        $this->setVar();
    }

    private function setVar() {

        $this->var = true;
    }
}

When would I use one over the other? I have seen many different php applications use the two different types. But there seems no general guideline to follow, or does it even matter for that fact?

Setters should set, not return. This should need no more explanation.

Neither of these look like correct property setters to me. A property setter should take a parameter, and assign the parameter value to the property:

class Simple {
    private $var;
    public function setVar($val) {
        $this->var = $val;
    }

    public function getVar() {
        return $this->var;
    }
}