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From the <a href="https://secure.php.net/manual/en/class.datetime.php#datetime.constants.cookie">official documentation</a>:<br/><br/>

<b>DateTime::ISO8601</b><br/>
<b>DATE_ISO8601</b><br/><br/>

ISO-8601 (example: 2005-08-15T15:52:01+0000)<br/>

<blockquote>
Note: This format is not compatible with ISO-8601, but is left this way for backward compatibility reasons.
Use DateTime::ATOM or DATE_ATOM for compatibility with ISO-8601 instead.
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