method question

I am looking to create a "comp-time" calculator of sorts using PHP/MySQL. I am still in the learning process of how all of this works. I want to be able to create a web interface where my employee can login and enter her comp-time as she works extra. I also want to be able to have an admin side where I can subtract her comp-time as she takes it and keep a running total of her remaining time. My question is - do you do the math in PHP or do you do it in MySQL. I am new to this type of logic so any input would be helpful. I am sure that there is a script like this that already exists but I want to try this one myself for the learning factor. thanks, DF