PHP serialize data case

Hi
Problem is, what to do, if users data is unknown. I mean, different fields and difefrent amount of them are needed to log some events.
I can use serialization of data, using one column as comments. May I will use it, when i dont know ,either data will be important or not for future purposes, and variable (field) amount is not clear for different users.

I heard, that some persons are using two fields in tables: ID and DATA



and data contains all serialized variables- fields. Example in PHP can be found

PHO serialize example

I am not sure am i right. I am not sure about performanve of such method

Can somebody comment?

Best regards