How to insert : and - into timestamp string from IoT device?

I have an Arduino IoT device which reads data from an electricity meter and
posts to a website database through a web call to a php script on my webserver.
THis works fine and the timestamp gets into the database as expected.
However it is hard to read so I want to add separators before storing onto the
database.

The timestamp retrieved from the meter looks like this example:
221216085802

It is sent by adding parameter &timestamp=221216085802 to a get call to the php
script.

In php on the webserver before I store it into the database I want to convert it
to a more readable format so it looks like this:

22-12-16 08:58:02

How can I do this in php code?

I.e insert two - one space and two : into the string at given positions


-- 
Bo Berglund
Developer in Sweden