1, borrowing records table, add "scheduled return date" field, and in each borrowing operation automatically fill the field.

2, the user table, increase the "lock date", in the return of books if beyond the "scheduled return date", the user table will be "locked date" filled with the current date + 5 days.

3. When borrowing a book, first determine whether the "lock date" is empty (if empty, then normal borrowing), if not empty, then determine whether the "lock date" is greater than the current date, and if it is, then refuse to borrow the book. If the current date is greater than the "lock date", the lock period has passed, you can borrow normally.

The ideas given above should solve your problem!