I may start with more of an integration-level test, as the application's custom framework provides a way to run API calls from a CLI syntax, making it somewhat easy to set up tests that emulate web requests without actually having to bother with HTTP, etc. Since the majority of the app consists of API calls with JSON responses, I figure the 80/20 rule will likely apply as far as covering a good chunk of the functionality.
Next up, making the PHP version upgrade a priority -- for which that test suite would be nice to have. 🙂