Can a WordPress site be converted into an iOS app? Maybe using something like PhoneGap to encapsulate the php?
No you can't encapsulate it.
You can crept APIs for your content or implement a theme that is responsive and looks good on mobile web but there is no way of directly wrapping your existing code.
Also, a part from the technical side, Apple always tries to reject apps that are purely copies of websites.
Please make good apps, that do something.