I don't think you can use the iPhone charging port as a digital out port. I use the headphone jack with a mini jack -> RCA adaptor but that's analog output.
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you can use your iphone's lightning as a USB out I use an Audioquest Red DAC plugged into the apple lightning camera adapter so I can stream and charge at the same time. work great for what it is (not really hi rez though) you need to plug into your usb input on your DAC. Hopefully that will work, if not then your DAC may not recognise the iPhone.