Does deleting a photo from iCloud on PC affect its copy on iPhone storage?
If the only copy on the phone is in Photo Stream or iCloud Photo Library then yes, if you delete it somewhere, it will be removed everywhere you are signed in to iCloud.

If you haven't subscribed to iCloud Photo Library, then you may still have the Camera Roll in the photos app, in this case the photo would remain in there, if it was taken on the iPhone originally.