How to POST JSON Data With PHP cURL?

Here is my code,

$url = 'url_to_post';
$data = array(
    "first_name" => "First name",
    "last_name" => "last name",
    "email"=>"[email protected]",
    "addresses" => array (
        "address1" => "some address",
        "city" => "city",
        "country" => "CA",
        "first_name" =>  "Mother",
        "last_name" =>  "Lastnameson",
        "phone" => "555-1212",
        "province" => "ON",
        "zip" => "123 ABC"
    )
);
$data_string = json_encode($data);
$ch=curl_init($url);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, array("customer"=>$data_string));
curl_setopt($ch, CURLOPT_HEADER, true);
curl_setopt($ch, CURLOPT_HTTPHEADER,
    array(
        'Content-Type:application/json',
        'Content-Length: ' . strlen($data_string)
    )
);

$result = curl_exec($ch);
curl_close($ch);

And at other page, I am retrieving post data.

    print_r ($_POST);

Output is

HTTP/1.1 200 OK
Date: Mon, 18 Jun 2012 07:58:11 GMT
Server: Apache
X-Powered-By: PHP/5.3.6
Vary: Accept-Encoding
Connection: close
Content-Type: text/html

Array ( ) 

So, I am not getting proper data even at my own server, it’s empty array. I want to implement REST using json as at http://docs.shopify.com/api/customer#create