Description:
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Following the documentation, DATE_ISO8601 contains this syntax: 2005-08-15T15:52:01+0000

Also, date('c') documents that: ISO 8601 date (added in PHP 5)  	2004-02-12T15:19:21+00:00

We should keep consistency when supporting this spec.

I propose the change then to DATE_ISO8601 constant, defined in ext/date/php_date.c, line 714:

- #define DATE_FORMAT_ISO8601 "Y-m-d\\TH:i:sO"
+ #define DATE_FORMAT_ISO8601 "Y-m-d\\TH:i:sP" 

Reproduce code:
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echo DATE_ISO8601 === date('c') ? 'true' : 'false';

Expected result:
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true

Actual result:
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false