MySQL Insert语句未执行

我正在尝试运行mySQL插入语句，如下所示：

function insertAppointment($connection, $id, $firstname, $lastname, $email, $phone, $date, $time){
                $sql = "INSERT INTO `appointments` (firstname, lastname, email, phone, app_date, app_time) VALUES ('" . $id . "', '" . $firstname . "', '" . $lastname . "', '" . $email . "', " . $date . ", " . $time . ")";
                $connection->query($sql);
        }

$connection是我的连接字符串，这不是问题所在。我可以用它来选择这样的语句：

function getTakenDates($connection){
                $query = mysqli_query($connection, "SELECT app_date, app_time FROM `appointments`");
                $results = array();
                while($row = mysqli_fetch_assoc($query)){
                        $results[] = $row;
                }
                return $results;
        }