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)