How would that work? How could I set up, for instance, my iPhone to port knock to allow port 53 access to the server from an unknown remote ip?

Edit: found this guide https://www.inmotionhosting.com/support/website/ssh/how-to-u...

Looks like I could just create some bookmarks for the ports and open them sequentially.

I think overall this is a much better solution than messing with php/bash - good idea and thanks!