Transfer favorites from cookie to database at login

Hi everybody,

At the moment the favorites saved in a cookie are lost when the user logs in.
I try to transfer the favorites at login from the cookie to the database.

I found a few older posts regarding this topic, but the recommended solutions don’t seem to work.
Has anybody found a way to transfer the favorites at login?

I really would appreciate some help since my php ain’t to good. Thanks in advance.

Thomas