When it comes to the AP Style of writing, there are some strict rules that make some situations difficult. But as long as you follow the rules to the letter, you should be fine. In this style, you should ALWAYS capitalize the first letter in a complete sentence. Therefore, iPhone should be “IPhone” when it starts the sentence.
Why Does it Matter?
It matters for two reasons. The first reason could be due to your audience. They may be expecting you to format your words correctly. One of the most popular scenarios would be for a school paper. The second reason if because spelling, grammar, and proper writing is a reflection of you as a person. You can tell a lot about a person about they way they communicate, not just the content.
Congratulations. You know one more thing than you did two minutes ago. IPhone is correct when it starts the sentence and iPhone ir correct in the middle.