Update fields with PHP session variables

Hello!

I want to create a system to track when a record is created and modified. I have created all of the fields needed and made them read only so they can't be changed, I just can't seem to figure out the rest.

I'm using a ON UPDATE CURRENT_TIMESTAMP attribute for lastmodified, however I still need some way to handle the other three options.

I need to pass $_SESSION["username"] to the field createdby and the current time to createdat when a new entry is submitted.

I also need to pass $_SESSION["username"] to the field modifiedby whenever a change is made to a field.

Any suggestions on how this can be done easily?