I have an array of objects, and i want to sum value of one of the property.Here is a picture which will show the structre of array.
        
This is working on lates PHP versions (tested on 7.2)
$sum = array_sum(array_column($res->intervalStats, 'spent'));
$sum = 0;
$result=$res->intervalStats;
foreach($result as $key=>$value){
if(isset($value->spent))   
    $sum += $value->spent;
}
echo $sum;
Make use of array_reduce function like below
$sum = array_reduce($res->intervalStats, function($i, $obj)
{
    return $i += $obj->spent;
});
echo $sum;
Sample Test
 [akshay@localhost tmp]$ cat test.php
 <?php
 $res = (object)array( "intervalStats" => array( (object)array("spent"=>1),(object)array("spent"=>5) ) );
 $sum = array_reduce($res->intervalStats, function($i, $obj)
 {
     return $i += $obj->spent;
 });
 // Input
 print_r($res);
 // Output
 echo $sum;
 ?>
Output
 [akshay@localhost tmp]$ php test.php
 stdClass Object
 (
     [intervalStats] => Array
         (
             [0] => stdClass Object
                 (
                     [spent] => 1
                 )
             [1] => stdClass Object
                 (
                     [spent] => 5
                 )
         )
 )
 6