Just to be clear, to use any non-gregorian calendar:
a. The locale must be in the form of locale@calendar=calendar-name
b. The calendar must use the intlDateFormatter::TRADITIONAL constant, or an intlCalendar object created with a locale as specified in a.
The calendar name can be 'gregorian', even if the intlDateFormatter::TRADITIONAL constant is used, which makes for a more generic code approach.