receive json post with php

Please forget my bad english. Im trying to receive a json post a payment interface website. But I cant decode it. When I print

echo $_POST;

I get:

Array

I get nothing When I try this:

if ( $_POST ) {
    foreach ( $_POST as $key => $value ) {
        echo "llave: ".$key."- Valor:".$value."<br />";
    }
}

I get Nothing when I try this:

$string = $_POST['operation'];
$var = json_decode($string);
echo $var;

I get NULL When I try this:

$data = json_decode( file_get_contents('php://input') );
var_dump( $data->operation );

The json format is (according to payment site documentation)

{
   "operacion": {
       "tok": "[generated token]",
       "shop_id": "12313",
       "respuesta": "S",
       "respuesta_details": "respuesta S",
       "extended_respuesta_description": "respuesta extendida",
       "moneda": "PYG",
       "monto": "10100.00",
       "authorization_number": "123456",
       "ticket_number": "123456789123456",
       "response_code": "00",
       "response_description": "Transacción aprobada.",
       "security_information": {
           "customer_ip": "123.123.123.123",
           "card_source": "I",
           "card_country": "Croacia",
           "version": "0.3",
           "risk_index": "0"
       }
    }
}

The payment site log says everything its ok. Could you help me? Thanks a lot!

Update> When I do

$data = json_decode(file_get_contents('php://input'), true);
var_dump($data);

I get

NULL