The following patch implements the logic to clone DatePeriod and DateInterval objects and also includes a test case:

Index: ext/date/php_date.c
===================================================================
--- ext/date/php_date.c	(revision 293574)
+++ ext/date/php_date.c	(working copy)
@@ -2213,7 +2213,9 @@
 	
 	zend_objects_clone_members(&new_obj->std, new_ov, &old_obj->std, Z_OBJ_HANDLE_P(this_ptr) TSRMLS_CC);
 
-	/** FIX ME ADD CLONE STUFF **/
+	new_obj->diff = timelib_rel_time_clone(old_obj->diff);
+	new_obj->initialized = 1;
+
 	return new_ov;
 }
 
@@ -2283,7 +2285,27 @@
 	
 	zend_objects_clone_members(&new_obj->std, new_ov, &old_obj->std, Z_OBJ_HANDLE_P(this_ptr) TSRMLS_CC);
 
-	/** FIX ME ADD CLONE STUFF **/
+	new_obj->start = timelib_time_ctor();
+	*new_obj->start = *old_obj->start;
+	if (old_obj->start->tz_abbr) {
+		new_obj->start->tz_abbr = strdup(old_obj->start->tz_abbr);
+	}
+	if (old_obj->start->tz_info) {
+		new_obj->start->tz_info = old_obj->start->tz_info;
+	}
+	new_obj->end = timelib_time_ctor();
+	*new_obj->end = *old_obj->end;
+	if (old_obj->end->tz_abbr) {
+		new_obj->end->tz_abbr = strdup(old_obj->end->tz_abbr);
+	}
+	if (old_obj->end->tz_info) {
+		new_obj->end->tz_info = old_obj->end->tz_info;
+	}
+	new_obj->interval = timelib_rel_time_clone(old_obj->interval);
+	new_obj->recurrences = old_obj->recurrences;
+	new_obj->include_start_date = old_obj->include_start_date;
+	new_obj->initialized = 1;
+
 	return new_ov;
 }

Index: ext/date/tests/bug50559.phpt
===================================================================
--- ext/date/tests/bug50559.phpt	(revision 0)
+++ ext/date/tests/bug50559.phpt	(revision 0)
@@ -0,0 +1,131 @@
+--TEST--
+Bug #50559 (Clone is not implemented for DateInterval and DatePeriod)
+--FILE--
+<?php
+date_default_timezone_set('Asia/Calcutta');
+
+# Test DateInterval cloning
+$dateInterval1 = new \DateInterval('P1D');
+$dateInterval2 = clone $dateInterval1;
+echo "============================\n";
+echo "DateInterval (original)\n";
+var_dump($dateInterval1);
+echo "============================\n";
+echo "DateInterval (clone)\n";
+var_dump($dateInterval2);
+
+# Test DatePeriod cloning
+$begin = new DateTime('2007-12-31');
+$end = new DateTime('2009-12-31 23:59:59');
+$interval = DateInterval::createFromDateString('last thursday of next month');
+$datePeriod1 = new \DatePeriod($begin, $interval, $end,
+                               DatePeriod::EXCLUDE_START_DATE);
+$datePeriod2 = clone $datePeriod1;
+
+echo "============================\n";
+echo "DatePeriod (original)\n";
+foreach ($datePeriod1 as $p) {
+	echo $p->format("l Y-m-d H:i:s\n");
+}
+
+echo "============================\n";
+echo "DatePeriod (clone)\n";
+foreach ($datePeriod2 as $p) {
+	echo $p->format("l Y-m-d H:i:s\n");
+}
+echo "============================\n";
+?>
+--EXPECT--
+============================
+DateInterval (original)
+object(DateInterval)#1 (8) {
+  ["y"]=>
+  int(0)
+  ["m"]=>
+  int(0)
+  ["d"]=>
+  int(1)
+  ["h"]=>
+  int(0)
+  ["i"]=>
+  int(0)
+  ["s"]=>
+  int(0)
+  ["invert"]=>
+  int(0)
+  ["days"]=>
+  int(0)
+}
+============================
+DateInterval (clone)
+object(DateInterval)#2 (8) {
+  ["y"]=>
+  int(0)
+  ["m"]=>
+  int(0)
+  ["d"]=>
+  int(1)
+  ["h"]=>
+  int(0)
+  ["i"]=>
+  int(0)
+  ["s"]=>
+  int(0)
+  ["invert"]=>
+  int(0)
+  ["days"]=>
+  int(0)
+}
+============================
+DatePeriod (original)
+Thursday 2008-01-31 00:00:00
+Thursday 2008-02-28 00:00:00
+Thursday 2008-03-27 00:00:00
+Thursday 2008-04-24 00:00:00
+Thursday 2008-05-29 00:00:00
+Thursday 2008-06-26 00:00:00
+Thursday 2008-07-31 00:00:00
+Thursday 2008-08-28 00:00:00
+Thursday 2008-09-25 00:00:00
+Thursday 2008-10-30 00:00:00
+Thursday 2008-11-27 00:00:00
+Thursday 2008-12-25 00:00:00
+Thursday 2009-01-29 00:00:00
+Thursday 2009-02-26 00:00:00
+Thursday 2009-03-26 00:00:00
+Thursday 2009-04-30 00:00:00
+Thursday 2009-05-28 00:00:00
+Thursday 2009-06-25 00:00:00
+Thursday 2009-07-30 00:00:00
+Thursday 2009-08-27 00:00:00
+Thursday 2009-09-24 00:00:00
+Thursday 2009-10-29 00:00:00
+Thursday 2009-11-26 00:00:00
+Thursday 2009-12-31 00:00:00
+============================
+DatePeriod (clone)
+Thursday 2008-01-31 00:00:00
+Thursday 2008-02-28 00:00:00
+Thursday 2008-03-27 00:00:00
+Thursday 2008-04-24 00:00:00
+Thursday 2008-05-29 00:00:00
+Thursday 2008-06-26 00:00:00
+Thursday 2008-07-31 00:00:00
+Thursday 2008-08-28 00:00:00
+Thursday 2008-09-25 00:00:00
+Thursday 2008-10-30 00:00:00
+Thursday 2008-11-27 00:00:00
+Thursday 2008-12-25 00:00:00
+Thursday 2009-01-29 00:00:00
+Thursday 2009-02-26 00:00:00
+Thursday 2009-03-26 00:00:00
+Thursday 2009-04-30 00:00:00
+Thursday 2009-05-28 00:00:00
+Thursday 2009-06-25 00:00:00
+Thursday 2009-07-30 00:00:00
+Thursday 2009-08-27 00:00:00
+Thursday 2009-09-24 00:00:00
+Thursday 2009-10-29 00:00:00
+Thursday 2009-11-26 00:00:00
+Thursday 2009-12-31 00:00:00
+============================

This patch causes issues. If I try the attached script I end up in an infinite loop.

Did you forget to attach the script?

on windows 7 + apache2 + php 5.3.6

apache crashes when

$cl = clone $dateIntervalObject;
in any scope, in any function, on an empty page too.

i tried to provide a backtrace but something goes wrong

i can't convert the generated .dmp file to a readable dump to attach here

If I swap lines 2 and 3 in the bug description code, this way:

$dateInterval1 = new DateInterval('P1D');
var_dump($dateInterval1);
$dateInterval2 = clone $dateInterval1;
var_dump($dateInterval2);

... then $dateInterval2 is not empty (as displayed by var_dump()). However, when 
I try to use it:

$d = new DateTime();
$d->add($dateInterval2);

... then I get "Warning: DateTime::add(): The DateInterval object has not been 
correctly initialized by its constructor in..."

(PHP 5.4.11 on Mac OS 10.7.5)

There's a WORKAROUND.
If you var_export the instance, then it won't be empty anymore, and you can clone it. Consider the following code:

<?php
header('Content-Type:text/plain');

$originalInstance = new DateInterval('P1D');
// Original instance is not initialized yet.
// You cannot clone it now, otherwise you'll get an empty copy.

$copyCreatedBeforeOriginalInstanceIsInitialized =
    clone $originalInstance;
echo PHP_EOL, 'This copy is empty: ';
var_export($copyCreatedBeforeOriginalInstanceIsInitialized);

// Now, let's initialize original instance by var_export'ing it.
// We don't want to output the var_export's result -
// it is used only to initialize our instance, not to output it.
$doNotEchoVarExport = true;
var_export($originalInstance, $doNotEchoVarExport);
// That's all! For now, original instance is already initialized,
// and we may clone it — we'll get correct copy.

$copyCreatedAfterOriginalInstanceIsInitialized =
    clone $originalInstance;

// Now, let's see what we've got:

echo PHP_EOL, 'Original instance is OK: ';
var_export($originalInstance);

echo PHP_EOL, 'Empty copy is still empty: ';
var_export($copyCreatedBeforeOriginalInstanceIsInitialized);

echo PHP_EOL, 'Correct copy is OK: ';
var_export($copyCreatedAfterOriginalInstanceIsInitialized);

// DateInterval behaves like Schrödinger's cat:
// the action of observation affects the phenomenon being observed.
// (var_export'ing the state of DateInterval affects its state.)
?>

The code above produces the following output:

This copy is empty: DateInterval::__set_state(array(
))
Original instance is OK: DateInterval::__set_state(array(
   'y' => 0,
   'm' => 0,
   'd' => 1,
   'h' => 0,
   'i' => 0,
   's' => 0,
   'weekday' => 0,
   'weekday_behavior' => 0,
   'first_last_day_of' => 0,
   'invert' => 0,
   'days' => false,
   'special_type' => 0,
   'special_amount' => 0,
   'have_weekday_relative' => 0,
   'have_special_relative' => 0,
))
Empty copy is still empty: DateInterval::__set_state(array(
))
Correct copy is OK: DateInterval::__set_state(array(
   'y' => 0,
   'm' => 0,
   'd' => 1,
   'h' => 0,
   'i' => 0,
   's' => 0,
   'weekday' => 0,
   'weekday_behavior' => 0,
   'first_last_day_of' => 0,
   'invert' => 0,
   'days' => false,
   'special_type' => 0,
   'special_amount' => 0,
   'have_weekday_relative' => 0,
   'have_special_relative' => 0,
))

The previous example doesn't work. The cloned copy can be var_export'ed correctly, that's true. But when you try to add it to a DateTime instance, the datetime won't change at all. Thus, cloned copies are broken. That's why I use another workaround:
<?php
class CloneDateInterval
{
    /**
     * @param \DateInterval $originalInstance
     * @return \DateInterval
     */
    public static function buildClone(\DateInterval $originalInstance) {
        $copy = new \DateInterval('P0D');
        foreach ($originalInstance as $propertyName => $propertyValue) {
            $copy->{$propertyName} = $propertyValue;
        }
        return $copy;
    }
}
?> 
This one works fine:
$copy = CloneDateTime::buildClone($originalDateInterval);
$datetime->add($copy); //OK

Also, if you don't wanna deal with manual coping all the fields like in example above, you may copy a \DateInterval object indirectly, using the inverse operations \DateTime::add() and \DateTime::diff() as shown below:

<?php
class CloneDateInterval
{
    /**
     * @var \DateTime
     */
    private static $midnight;

    private static function getMidnight(): \DateTime
    {
        if (self::$midnight === null) {
            self::$midnight = new \DateTime('midnight');
        }
        return clone self::$midnight;
    }

    public static function makeClone(\DateInterval $dateInterval): \DateInterval
    {
        return self::getMidnight()->diff(self::getMidnight()->add($dateInterval));
    }
}
?>

Although, maybe this method is not as computationally efficient as the one from above, but it definitely is an interesting alternative, at least from theoretical point of view. :D

Closing as it was implemented via http://git.php.net/?p=php-src.git;a=commit;h=48598a23.

https://3v4l.org/HA8kn