How can I stop Photos from opening when I plug in my iPhone in iOS 11?????

If it is Photos on the computer, that is the expected behavior. You can change that in Photos on the computer. I can't access mine right now to provide more detailed instructions, but I think if you select the phone in the sidebar, you can make the change. Also check Photos/Preferences.

In MacOS High Sierra, you need to click on your device in the left-hand column then you should see a box you can untick towards the middle of the screen:

What do you mean it opens? Plugging the device and leaving it connected to wifi will upload your photos to iCloud. The Photos app shouldn't open automatically.

That's quite odd as when I plug my iPhone (into an electrical outlet), my Photos app doesn't automatically launch.

Are you plugging it into a computer?

There is a way to change it to not open. Since I don't have access to mine right now, I can't provide better instructions.