You have to read your object.
It says that it's a standard object who contains a standard object who contains an array. So, first tell about first object than second object to end with the array's name followed by key you want to get :
$some_obj->HelloWorldResult->string[0]
For an example :
$some_obj = new stdClass();
$some_obj->HelloWorldResult = new stdClass();
$some_obj->HelloWorldResult->string = array(
0,
4546330305913,
"1395/11/20",
0
);
print_r($some_obj);
Output :
stdClass Object
(
[HelloWorldResult] => stdClass Object
(
[string] => Array
(
[0] => 0
[1] => 4546330305913
[2] => 1395/11/20
[3] => 0
)
)
)
Then to access some value :
var_dump($some_obj->HelloWorldResult->string[0]);
Output :
int(0)