Description:
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The result if compare operator on two DateInterval objects is unpredictable.
See code in the "test script" section:
the result of the if() statement depends on the code before. If, for instance, you print the DateInterval, the internal structure seems to change, and the result is correct. Otherwise, the result is not correct. The system should output an error or a warning if the compare operater is not fully implemented.

Test script:
---------------
<?php
$old = DateTime::createFromFormat('Y-m-d H:i:s', '2018-11-26 09:29:00'); 
// constant here for demo only, i read datetime field from database
$now = new DateTime('now',new DateTimeZone('Europe/Berlin'));
$maxdiff = new DateInterval('PT30M');
$actualDiff = $now->diff($old);

// echo "<pre>"; print_r($now); echo "</pre><br />";
// echo "<pre>"; print_r($maxdiff ); echo "</pre><br />";

if ($actualDiff>$maxDiff) echo "time is up" else echo "just wait";
?>


Expected result:
----------------
throw an error if an nvalid operator ist used 
minimum request: add information about this behauviour into documentation of DateInterval

Actual result:
--------------
operator is performed, result is unpredictable