For myself, I just encode it in the url and use $_GET on the destination page. Here's a line as an example.
$ch = curl_init();
$this->json->p->method = "whatever";
curl_setopt($ch, CURLOPT_URL, "http://" . $_SERVER['SERVER_NAME'] . $this->json->path . '?json=' . urlencode(json_encode($this->json->p)));
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$output = curl_exec($ch);
curl_close($ch);
EDIT: Adding the destination snippet... (EDIT 2 added more above at OPs request)
<?php
if(!isset($_GET['json']))
die("FAILURE");
$json = json_decode($_GET['json']);
$method = $json->method;
...
?>