$mysqli = $this->DbConnect();
$sql = "UPDATE `tbl` SET `campo1` = ?, `campo2` = ? WHERE `condizione` = ?";
$stmt = $mysqli->prepare($sql) or trigger_error($mysqli->error, E_USER_ERROR);
$stmt->bind_param('sii', $campo1, $campo2, $condizione);
$stmt->execute() or trigger_error($stmt->error, E_USER_ERROR);
$mysqli->close();