Actual and expected result are switched (sorry for that).

I've found a similar issue: https://3v4l.org/TrPvp

What I found was that if I compared two DateInterval objects, they would always return false.

That is, until I actually get some data from one of the objects (in my case I just var_export($interval, true) and discard the output) at which time everything starts to return true. Also weird.

If I inspect both intervals first however, suddenly it all evaluates correctly! I've tested this in PHP 7.0.0 and PHP 7.0.6. 3v4l.org suggests that it will work back to 5.5.8 though

I believe I have run into the same issue. It appears a new DateInterval is considered "0", or perhaps not fully initialized internally. After "certain interactions" with the DateInterval, for example as an arg to print_r(), it appears to become fully initialized.

The provided example yields strange behavior in the two version of PHP at my disposal:

PHP 5.4.45 (cli) (built: Sep  4 2015 15:40:44) 
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2014 Zend Technologies
    with Xdebug v2.2.1, Copyright (c) 2002-2012, by Derick Rethans


PHP 5.5.36 (cli) (built: May 29 2016 01:07:06) 
Copyright (c) 1997-2015 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2015 Zend Technologies


Test Script
-----------

<?php
$x = new DateTime('2016-01-01 00:00:00 UTC');
$y = new DateTime('2016-01-01 00:00:15 UTC');

// Should be 0s
$z1 = $x->diff($x);
// Should be 15s
$z2 = $x->diff($y);
// Should be 30s
$z3 = new DateInterval('PT30S');

echo "No inspections\n";
var_dump($z1 == $z2);
var_dump($z1 == $z3);
var_dump($z2 == $z3);
var_dump($z1 > $z2);
var_dump($z2 > $z3);

echo "Inspect \$z1\n";
print_r($z1);
var_dump($z1 == $z2);
var_dump($z1 == $z3);
var_dump($z2 == $z3);
var_dump($z1 > $z2);
var_dump($z2 > $z3);

echo "Inspect \$z2\n";
print_r($z2);
var_dump($z1 == $z2);
var_dump($z1 == $z3);
var_dump($z2 == $z3);

// After inspecting all DateIntervals, these are the expected results.
echo "Inspect \$z3\n";
print_r($z3);
var_dump($z1 == $z2);
var_dump($z1 == $z3);
var_dump($z2 == $z3);
var_dump($z1 > $z2);
var_dump($z2 > $z3);



Script Output
-------------

No inspections
bool(true)
bool(true)
bool(true)
bool(false)
bool(false)
Inspect $z1
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 0
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 0
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)
bool(false)
bool(false)
bool(true)
bool(true)
bool(false)
Inspect $z2
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 15
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 0
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)
bool(false)
bool(false)
bool(false)
bool(false)
bool(true)
Inspect $z3
DateInterval Object
(
    [y] => 0
    [m] => 0
    [d] => 0
    [h] => 0
    [i] => 0
    [s] => 30
    [weekday] => 0
    [weekday_behavior] => 0
    [first_last_day_of] => 0
    [invert] => 0
    [days] => 
    [special_type] => 0
    [special_amount] => 0
    [have_weekday_relative] => 0
    [have_special_relative] => 0
)
bool(false)
bool(false)
bool(false)
bool(false)
bool(false)

https://github.com/php/php-src/pull/4039

This is fixed as of PHP 7.4.0[1], so I'm closing this ticket.

[1] <https://3v4l.org/bQnf0>