Getting NULL when reading JSON Object in PHP side

I'm trying to send a JSON object to PHP through ajax. But am getting null value on PHP side.

Jquery code

 t_data = [{"name":"John", "age":30, "car":null},{"name":"John", "age":30, "car":null}]      
      $.ajax({
        url: "someupdate",
        data: {"test":JSON.stringify(t_data)},
        dataType: "json",
        type: "POST",
        beforeSend: function () {
          startLoading('Some  Update in progress, please wait');
        },
        complete: function () {
          stopLoading();
        },
        success: function (data, textStatus, jqXHR) {
        },
        error: function (jqXHR, textStatus, errorThrown) {
        }
      });
    }

PHP Side

$request = $this->getRequest();
        if ($request->isPost()) {
            $configDetails = $this->params()->fromPost('test');
   var_dump($configDetails);
var_dump(json_decode($configDetails);

Output

string(153) "[{"name":"John","age":30,"car":null},{"name":"John","age":30,"car":null}]" NULL

As you can see in the output the response is visible in the required format when am not using json decode. I wonder why it showing null value when I decoded.

Could anyone kindly help me to identify what is missing my implementation for getting the result as expected

#UPDATE

1. I tried without JSON.stringify and got below as the output

array(4) { [0]=> bool(false) [1]=> bool(false) [2]=> bool(false) [3]=> bool(false) }

2. Someone could kindly suggest a betterway for passing the JSON object from Jquery to PHP (Zend Framework is what am using)
3. I'm actually creating an object using

function createJSON() {
    jsonObj = [];
    $("input[class=email]").each(function() {

        var id = $(this).attr("title");
        var email = $(this).val();

        item = {}
        item ["title"] = id;
        item ["email"] = email;

        jsonObj.push(item);
    });

    console.log(jsonObj);
}