php difference between two times in hours

If you are using 24 hours format you don't need AM or PM annotations at all.

By default 00:00:00 means 12:00 AM of the day. 

In the given example, from timestamp shows time = 12:00:00, it means 12 hours passed, so your time is - 12:00 PM of the day.

To timestamp shows 20:00:00, it means 8:00 PM of the respective day. Max time can be 23:59:59, and after that it will be turned to 00:00:00 (12:00 AM)

You can add number of hours in 12:00:00 PM to count further all hours in 24 hours format if you see the time > 12:00:00.

It shows you are using 24 hours format.

From date shows 12 O'clock of Afternoon

To date shows 8 O'clock of evening!