Description:
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ISO 8601 differentiates between local time (without time zone), UTC with letter "Z" and time offsets from UTC like "+02:00".
https://en.wikipedia.org/wiki/ISO_8601#Time_zone_designators

When formatting datetimes, it is sometimes desired to get the "Z" for UTC times and otherwise the offset.
But at the moment there are only the formatting chars "O" (for "+0200" etc.) and "P" (for "+02:00" etc.).

I suggest to add lower case "p" as another formatting char which behaves the same as upper case "P" but returns "Z" for UTC datetimes.

https://github.com/php/php-src/pull/5896

Test script:
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<?php

$datetime = new DateTime('2020-07-26 20:00:00', new DateTimeZone('UTC'));
$datetime->format('Y-m-d\TH:i:sP'); // '2020-07-26T20:00:00+00:00'
$datetime->format('Y-m-d\TH:i:sp'); // '2020-07-26T20:00:00Z'

$datetime = new DateTime('2020-07-26 20:00:00', new DateTimeZone('Europe/London'));
$datetime->format('Y-m-d\TH:i:sP'); // '2020-07-26T20:00:00+00:00'
$datetime->format('Y-m-d\TH:i:sp'); // '2020-07-26T20:00:00+00:00'

$datetime = new DateTime('2020-07-26 20:00:00', new DateTimeZone('Europe/Berlin'));
$datetime->format('Y-m-d\TH:i:sP'); // '2020-07-26T20:00:00+02:00'
$datetime->format('Y-m-d\TH:i:sp'); // '2020-07-26T20:00:00+02:00'