Turn off your Iphone without Power Button

Basically, to turn off an Iphone we have to do the following steps;

1. Hold down the sleep/wake button for a few seconds, until you see a message appear on the screen. This button is located on the right of the phone (it's either on the top or the side depending on your iPhone model).
2. A power button will appear onscreen that reads slide to power off. Move the slider all the way to the right to shut off the phone.
3. A progress wheel will appear in the center of the screen. The iPhone will turn off a few seconds later.

Actually, you can Turn off your Iphone without power button or in case the power button is broken you still can turn it off. 

Here is how it works by using AssistiveTouch;

1. Set Lock Screen button on AssistiveTouch
2. Press and hold Lock Screen for few second until Power icon slide appear then swipe the power icon from left to right across slide to power off to shut down your iPhone.

Please watch the video below for detail.