Help me with a PHP function

I'm using a source code from github, used to book appointments and store them on database. Unfortunately, the client can't choose how long the service will last, so let's say. If he needs 2 hours and a half for that service, he can't do that, because when we,as fornitors add new services, we decide how minutes that service lasts. What I want is, a dropdown menu where the client selects how many hours he wants for that service, the price duplicates(in case of 2 hours), it gets stored in the database and obviously at the calendar and moreover, when a new client comes by, he can't book an appointment at that time (well, that's obvious) Like, I said, the source code of this PHP+SQL is already on github.