Margarita

Just got back from an extended trip to NYC. Macy Day Parade was exciting.
Keep up the asking questions. I have sure asked my share.
Glad you found a solution. There is always something new to learn.

Your last question:
I think the crux of your question is - If running some process - how to determine if somone has visited (and maybe more importantly visited and changed data) since the last process was run.

If that is the case: (if not, maybe a little more details)

A table design can include a timestamp field. I typically include two. One for the createion data/time and one for the Last updated date/time.
The default for the created is NOW() (or now)
The table for the last updated - set the value in a hidden field to Now

If the question can be a little more focused - I bet you will get a lot of great advice.
Best Regards

You are doing great! good questions. Sorry I have to catch the train and can't look into it more.