How much experience do you have? Building this project from scratch with no PHP experience would be a huge first project, you'd risk making security mistakes, and you wouldn't be able to plan a schedule. If you don't have much PHP experience, it would make more sense to buy and install a pre-made appointment booking system.
If you are going to build it yourself, what made you decide to use XML? So far, I've found that XML is great when I need to communicate with other backend systems... but when it's all in house, there's not much need. And what made you decide to use PHP for that matter?