There is no official "developer mode" on the iPhone, but there are a number of ways to access hidden features and settings on the device. One popular method is to jailbreak the iPhone, which gives you root access to the file system and allows you to install custom applications and tweaks. Another way to access hidden features is to use secret codes that can be entered into the phone's dialer app. These codes can be used to access various menus and functions, including a hidden testing menu.