Description:
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I have tested this bug on Windows / Linux and PHP versions 5.5 and 7.0. The DateTime::createFromFormat function does not correctly use the ISO 8601 modifier ('c'). Trying to parse an ISO 8601 datetime string using this function results in 'false'.

Test script:
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<?php
// Create datetime
$date = new \DateTime('now');
var_dump($date); // = DateTime Object

// Convert to string
$fDate = $date->format('c');
var_dump($fDate); // = ISO 8601 string

// Read data to object
$rDate = \DateTime::createFromFormat('c', $fDate);
var_dump($rDate); // = false

Expected result:
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\DateTime::createFromFormat('c', $fDate); should result in a valid DateTime object, with the same contents as the original $date object.

Actual result:
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False, the createFromFormat function cannot parse the string