A global incrementor service in Phalcon

I try to create a service in Phalcon, that only increments a number.

My Class:

<?php
class FormCounter {

    private $counter = 0;

    public function increment() {
        $this->counter++;
        return $this->counter;
    }

}

In services:

$di->set('formCounter', function () {
    return FormCounter();
});

In my Form Class I want to create an ID in every form with increments. The first form i create will have ID "form_1", second "form_2" etc.

$this->form_id = $this->getDi()->getFormCounter()->increment();

But it won't increment. It returns 1 everytime. How do I do this without using global variables?