How calculate the sum of two DateInterval object

I have two date interval object, is there any default method to add those interval object ?

$date1 = date_create("2013-03-15");
$date2 = date_create("2013-12-12");
$diff_1=date_diff($date1,$date2);
echo $diff_1->format("%y years").' '.$diff_1->format("%m months"). ' ' . $diff_1->format("%d days");
//0 years 8 months 27 days

$date3 = date_create("2015-02-15");
$date4 = date_create("2015-12-12");
$diff_2=date_diff($date3,$date4);
echo $diff_2->format("%y years").' '.$diff_2->format("%m months"). ' ' . $diff_2->format("%d days");
//0 years 9 months 27 days

$diff_1+$diff_2 = 1 year 6 months 24 days

What i need is to calculate the sum of diff_1 and diff_2?

The probably easiest way is to create a new object and cloning it, adding the two (or more) DateTimeIntervals (in your case $diff_1 and $diff_2) to the new object. By now finding the difference between the new object and its clone, is the sum of the two DateTimeIntervals you originally had.

// Define two intervals
$date1 = date_create("2013-03-15");
$date2 = date_create("2013-12-12");
$diff_1 = date_diff($date1,$date2);

$date3 = date_create("2015-02-15");
$date4 = date_create("2015-12-12");
$diff_2 = date_diff($date3,$date4);


// Create a datetime object and clone it
$dt = new DateTime();
$dt_diff = clone $result;

// Add the two intervals from before to the first one
$dt->add($diff_2);
$dt->add($diff_1);

// The result of the two intervals is now the difference between the datetimeobject and its clone
$result = $dt->diff($dt_diff);
var_dump($result);

Result of the dump includes

  ["y"]=>
    int(1)
  ["m"]=>
    int(6)
  ["d"]=>
    int(21)

..which is 1 year, 6 months and 21 days.

Live demo

Sidenote
You don't have to concat so many different formats with your format(). You can do it all in a single line,

echo $result->format("%y years %m months %d days");