如何更新一整天的数据库值

I am building a web application, for travelling. I have managed to get users to be able to insert how much they have spent on each category (i.e. travel, accomodation, food, etc) into the database once from a form. However, I want them to be able to contiously add to the total value of each category for just that day using the same form, and then everyday have a new total for each category as well. I'm not quite sure how I would do that at the moment. Here is my code so far for inserting the values into the database from my form (which works):I am building a web application, for travelling