OK, a possibility would be to check if the subject already begins with "re:".

that's the way it should be done.

If you change a username, the name is also changed in the entry-table. The reason for saving it in the entry-table is: if you delete a user, the postings would be nameless. This way, the names remain in the postings.

don't take it personally, but i think this procedure isn't very elegant.
why should an information, i already have, be saved to the db about 100 times more? isn't that data waste?
I'd prefer to add a new column to the user table, in which the user-status (deleted|banned|etc.) could be saved. even if the user-information is finally deleted from the db, what can be said against marking postings with "user deleted"?

But there isn't anymore a board-view in this version.

ok, for me one of the the advantages of your script was it's flexibility...
what are your reasons for this change?