How can I trigger PHP code on page load?

[SidCQ]

Complete newbie here. I made an app on my partner account, and I installed it on our store. I found in the store page itself you can create apps and install them but there doesn't seem to be a way to actually add code to them???

Anyway, I can make php files now. What I want to do is listen for POST data and write to a database like you would any normal php page out there on the internet.
How do I call this php code when a page loads?