Description:
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From manual page: http://www.php.net/class.dateperiod
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DatePeriod does not appear to support ISO 8601 date periods without an upper bound (e.g., 'R/2016-16-20T00:00:00Z/P7D') or null end dates. The ISO spec itself supports this, so a note should be made in the documentation that DatePeriod does not.

Test script:
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// This throws an exception with message 'DatePeriod::__construct(): Unknown or bad format (R/2016-06-20T00:00:00Z/P7D)'
$dp1 = new DatePeriod('R/2016-06-20T00:00:00Z/P7D');

// This silently accepts the null third argument; the resulting object has a null end date and one recurrence.
$dp = new DatePeriod(new DateTime('2016-06-20'), new DateInterval('P7D'), null);