Strictly speaking, "I have an iphone" is a simple statement about the present, while "I have got an iphone" is a statement about something that you did in the past, like saying "At some point in the past I got an iphone". Consider "I have kicked the ball" or "I have eaten the cake". "got" means "obtained" in this context.
However, this distinction is often ignored, so the two phrases have become somewhat interchangeable. That site says that they mean the same: I don't think this is quite true, formally speaking, but from a pragmatic point of view they would tend to mean the same thing. To me, "I have got an iphone" sounds slightly childish compared to "I have an iphone".