What do you mean "planning"? They already are in some places, including Apple Stores themselves. But more often you see iPads in that role with a bigger screen for customers. Are they doing the same behavior? (I don't see why not).

For the moment they have an external payment / card terminal; other than the card reader itself, I don't see why an iphone wouldn't work as a formal payment terminal.

It already does.

Part of the new PCI standard called "Software based PIN entry on COTS" [1] which is also known generically as "PIN on glass".

Banks are now offering it to one-person trade companies etc [2].

[1] https://blog.pcisecuritystandards.org/new-faqs-on-software-b...

[2] https://www.westpac.com.au/business-banking/merchants-and-pa...