php field formatter

Related to this and also maybe easiest to do it at once is to split the interval_apply_interval() function so that there is a function that generates an interval string (used for the php formatter) and a function that applies applies an interval string to a date.

That way the interval_apply_interval() could be used with other fields that contain a php date nterval string.

For example, #1921038-2: Recurring interval checkout pane could provide a recurring payment action that takes an interval string and uses interval_apply_interval() (as it currently does).
This would allow the recurring payment interval to be taken from an interval field (or other field containing an interval string) via a token.

Or interval_apply_interval() could remain as is and have a new function for applying an interval string (or just use DateTime::modify directly).

It's late so I could just be dreaming out loud but I will review this again tomorrow.

Yep, that's what I was planning to do.

There will be no entering php by anyone.

Patch will come tonight.

Patch ahoy.