how can i change a variable after submition in symfony 7

i m trying to change a variable after i click on a submit button after verifying some conditions.
i used normal html forms instead of symfony forms idk if there is any difference.
controller:

#[Route('/test', name: 'test')]
    public function test(Request $request): Response
    {
        $response = 'a';
        if ($request->isMethod('POST')){
            $response = "value changed";
        }
        return $this->render('charge/test.html.twig', [
            'response' => $response
        ]);
    }

template:

{% extends 'base.html.twig' %}

{% block title %}Hello !{% endblock %}

{% block body %}
    <div class="container">
        <h1>{{ response }}</h1>
        <form method="post">
            <input type="submit" name="sub1" value="sub1">
            <input type="submit" name="sub2" value="sub2">
        </form>
    </div>
{% endblock %}

Value of response never changes after clicking on any of the submit buttons.