you can put your json in a parameter and send it instead of put only your json in header:
$post_string= 'json_param=' . json_encode($data);
//open connection
$ch = curl_init();
//set the url, number of POST vars, POST data
curl_setopt($ch,CURLOPT_POST, 1);
curl_setopt($ch,CURLOPT_POSTFIELDS, $post_string);
curl_setopt($curl, CURLOPT_URL, 'http://webservice.local/'); // Set the url path we want to call
//execute post
$result = curl_exec($curl);
//see the results
$json=json_decode($result,true);
curl_close($curl);
print_r($json);
on the service side you can get your json string as a parameter:
$json_string = $_POST['json_param'];
$obj = json_decode($json_string);
then you can use your converted data as object.