There are a few undocumented dynamic properties, including one for the number of recurrences (which is calculated when using an end date).
https://3v4l.org/iGAV1

But don't go using them just yet.

HHVM implements a few properties in the class definition but makes them private: $start, $interval, $end, $options, $current, $recurrances [sic], and $iterKey. PHP's properties are created on the object and not defined on the class.
- ReflectionObject: https://3v4l.org/8BKTW
- ReflectionClass: https://3v4l.org/CfZuJ

Since there's already getDateInterval, getEndDate, and getStartDate, maybe making the properties real and private and creating a getRecurrences would be best.
(HHVM does not have a getRecurr[ae]nces method either)

To be honest one should definetly not rely on those undocumented properties because they are "broken" the interval property returns a DateInterval object in invalid state and the enddate/reccurences properties values sometimes are badly inferred.

That's why I thought adding getRecurrences was to be consider a bug fix since:

- it would give full access to the DatePeriod property in a standardized way.
- the others getter methods were added in a patch version of PHP5.6.
- i don't think an RFC is needed for this kind of patch given the other getter methods are already present and this addition just complement them