Description:
------------
Interacting with a copied DateTime variable (without using the reference '&' syntax) still changes the original variable.

Test script:
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<?php
// Get today's date.
$originalDate = date_create();;

// Take a copy.
$copiedDate = $originalDate;
// This *will* work for some reason : $copiedDate = clone $originalDate;

echo 'originalDate=' . date_format($originalDate, 'Y-m-d') . '<br/>';

// Add 1 day.
date_add($copiedDate, date_interval_create_from_date_string('1 days'));
// This doesn't work either : $copiedDate->add(date_interval_create_from_date_string('1 days'));

// $originalDate *SHOULD* still be the same.
echo 'originalDate=' . date_format($originalDate, 'Y-m-d') . '<br/>';

// This behaviour if desired is different with other objects:
$originalString = array('Sausages');
echo $originalString[0] . '<br/>';
$copiedString = $originalString;
$copiedString[0] = 'Ice Cream';
echo $originalString[0] . '<br/>';
?>

Expected result:
----------------
The $originalDate to remain the same as it was before $copiedDate was changed.

Actual result:
--------------
$originalDate changes.