Unique random number for every class instantiation

I want to generate a random ID via rand() for every instantiation of my class. And I want it to be unique. Here is what I wrote, but this doesn't work.

class Computer
{
    private $id = 0; // placeholder for ID
    private $id_list = []; // placeholder to store used IDs

    public function __construct()
    {
        $this->checkID();
    }

    private function checkID()
    {
        $this->id = rand(1,3);
        if (!in_array($this->id, $this->id_list))
        {
            array_push($this->id_list, $this->id);
        } else {
            $this->checkID();
        }
    }
}

I've tried this intentionally with generating low values, like rand(1,3), and I got three objects with IDs 2, 2, 1. So it's not working.

I assume that is because for every new instance of a class $id_list array becomes empty. But I've seen people doing almost the same thing. Declaring a $counter variable and putting $this->counter++ in the __construct method. So what's the difference?