Regarding https://www.php.net/manual/en/dateperiod.construct.php

The documentation says that ISO8601 repeating interval strings are supported, but does not reveal that time zone offsets are not supported.

Calling the documentation's example (with Z indicating UTC timezone) works:

$iso = 'R4/2012-07-01T00:00:00Z/P7D';
$period = new DatePeriod($iso);

The same string with an one-hour offset throws an exception:

$iso = 'R4/2012-07-01T00:00:00+0100/P7D';
$period = new DatePeriod($iso);
// throws "Uncaught Exception: DatePeriod::__construct(): Unknown or bad format (R4/2012-07-01T00:00:00+0100/P7D)"

Valid variations of the offset part throw an exception as well:

$iso = 'R4/2012-07-01T00:00:00+01/P7D';
$iso = 'R4/2012-07-01T00:00:00+01:00/P7D';

This fact should be included in the notes section