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step 1) pick up your digital camera
step 2) point at iPhone screen
step 3) click shutter button


just kiddin' 😀 I don't have an iPhone so I have no info on the topic.
 
Apple didn't include a screen capture utility, so your phone has to be jailbroken.

Then Google "iphone screen capture" for several solutions from onscreen apps to command line utilities.
 
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