Description:
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(This is really a feature request, rather than a bug per se.)

Unlike DateTime objects, DateInterval objects cannot easily be compared within PHP. While it would be possible to concoct a workaround in userspace using a few calls to DateInterval::format() and some arithmetic, it would probably be preferable to implement it within ext/date itself.

I've prepared a patch (yes, it even has a simple test) against PHP_5_3 at http://www.adamharvey.name/patches/DateInterval-comparators.patch which implements comparator support. I can probably prepare a HEAD patch if necessary; I just don't have a HEAD checkout to hand to do so at present.

There's one fairly significant issue with this patch worth noting: I've implemented a new function (timelib_rel_time_to_seconds) which converts a timelib_rel_time structure to the number of seconds it represents. The issue with this is that, as per bug #49778, we don't always know exactly how many days a timelib_rel_time actually represents because of the varying number of days in a month and year.

As per the comments in the function, for now I've fudged it and effectively chosen the default length of a month and year out of thin air if the days field isn't set within the structure. If the resolution to bug #49778 results in the days field always being filled in, then timelib_rel_time_to_seconds can be simplified accordingly. Alternatively, we could only support this in cases where we definitely know the days within the timelib_rel_time structure and error out otherwise.

As a side note, I have a second patch that I can upload that implements support for a DateInterval::getSeconds() method which effectively provides a PHP wrapper around the proposed internal timelib_rel_time_to_seconds function. If it's decided to accept this approach, I can create another bug to track getting that in.

Reproduce code:
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<?php
$ten = new DateInterval('PT10S');
$twenty = new DateInterval('PT20S');
var_dump($ten < $twenty);
?>

Expected result:
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bool(true)

Actual result:
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bool(false)