Вы должны попробовать json_decode () в PHP:
$test='{"var1":null,"var3":null,"status":{"code":150,"message":"blah blah"}}';
$responseObj = json_decode($test);
echo $responseObj->status->code;
Для класса PEARS Services_JSON ( Документация ):
// create a new instance of Services_JSON
$jsonService = new Services_JSON();
$test='{"var1":null,"var3":null,"status":{"code":150,"message":"blah blah"}}';
$jsonService->decode($test);
echo $responseObj->status->code;