How do I handle json data sent as an HTTP Post to a cakephp app?

if($this->RequestHandler->requestedWith('json')) {
    if(function_exists('json_decode')) {
        $jsonData = json_decode(utf8_encode(trim(file_get_contents('php://input'))), true);
    }

    if(!is_null($jsonData) and $jsonData !== false) {
        $this->data = $jsonData;
    }
}

This is a codesnippet which was proposed to be in the core, see https://trac.cakephp.org/ticket/6125. Maybe it's what you're looking for.

-- Bjorn

You can use these simplest way :

$data = $this->request->input ( 'json_decode', true) ;