Building An App Question by Ben

Hello,

I have been reading through the Shopify help sections and strangely found a whole in the entire explantion of Shopify Apps.

I get how to join up, start a store, get my api key... But how do I actually make the app?  I know Liquid, CSS, html, and some php...

But where is the envirntment that I enter the code to call up the Shopify data?  What language is it in?

Thanks.

-Ben