Cosa cambia in PHP 8.0.1 su date

[ffw]

Ragazzi, mi trovo un modify->('+1 day') che non funziona in php 8
Qualcuno vede cosa sbaglio ?

                if(isset($_POST['grafico']))                {                    $star_time =    new DateTime($_POST['from']);                    $stop_time =    new DateTime($_POST['togo']);                }                else                {                    $star_time =    new DateTime($start);                    $stop_time =    new DateTime($date);                }                echo $star_time->format('d/m/y')." - ".$stop_time->format('Y-m-d');                 for($i = $star_time; $i < $stop_time; $i->modify('+1 day') )                {                    $labels[] = gdate($i->format("Y-m-d"));                    $iDay = $i->format("Y-m-d 00:00:00");                    $fDay = $i->format("Y-m-d 23:59:59");
                    $stmt = $indexPage->runQuery("SELECT id, nome, cognome FROM UTENTI WHERE level > 1 ORDER BY id ASC");                    $stmt->execute();                    while($adm = $stmt->fetch(PDO::FETCH_ASSOC))                    {                        $stmt_contacts = $indexPage->runQuery("SELECT id FROM CONTACTS WHERE risp='R' && cDate BETWEEN '$iDay' AND '$fDay' && user=:usr");                        $stmt_contacts->execute(array(":usr"=>$adm['id']));
                        if(!in_array($adm['id'], $user))                        {                            $user[$adm['id']] = $adm['id'];                            $background[$adm['id']] = "rgb(".rand (0, 255).", ".rand (0, 255).", ".rand (0, 255).")";                            $nome[$adm['id']] = $adm['cognome']." ".$adm['nome'];                        }                        $count[$adm['id']] .=  $stmt_contacts->rowCount();                                                }                }