i am currently working on codeigniter project.i have a database name is `"student_detail"` the table value have like this

```
  payment_date
1 NULL
2 NULL
3 2017-12-06 08:47:40

payment_date column is a DATETIME format

```

. when i print this column it returns like this

```
payment_date
1 1970-01-01 00:00:00
2 1970-01-01 00:00:00
3 2017-12-06 08:47:40

```

but i need to return as NULL not default time format.