Yes, it is built into iOS - swipe down from the top right corner and chose mirror screen, this will mirror your screen on a compatible device (I only tested with a MacBook).

As other comment mentions below, this does not allow controlling the device.

You can use QuickTime on MacOS to see your iPhone as a video. Not sure other platforms.

The difference is that you can't control the phone from QuickTime, you can only view it. That is a pity because it is easier to control the phone with a mouse when developing apps (less tiring).