Is the iPhone always using Wifi (instead of cellular data network) when connected? When I'm in range and connected to particular Wifi, having cellular data transmission enabled at the same time, will the iPhone always use the Wifi connection?

Or can the app developer choose which network has priority (e.g. always use cellular data transmission and not Wifi)?
I believe you cannot chose programmatically whether to use the WiFi or the cellular data connection.

If the iPhone has a WiFi network available, it will automatically use it, otherwise, it will fallback on the cellular data connection.