Check for $ _POST fields in a POST method form

if you want to check username and password fields, try below code

if (isset($_POST['submit-btn'])) {
    $username = filter_var($_POST['username'], FILTER_SANITIZE_STRING);
    $password = filter_var($_POST['password'], FILTER_SANITIZE_STRING);
    if(empty($username))
    {
        echo "Username empty"; return false;
    }
    if(empty($password))
    {
        echo "Password empty"; return false;
    }

    if($username && $password)
    {
        $login_array = array();
        $login_array['user_login'] = $username;
        $login_array['user_password'] = $password;

        $verify_user = wp_signon($login_array, true);
        if (!is_wp_error($verify_user)) {
            echo "<script>window.location = '" . site_url() . "'</script>";
        } else {
            echo 'Credenziali non valide!';
        }
    }
}

let me know if this works for you!