PHP REST server - AJAX - JSON

Been going around in circles trying to get an AJAX call to a PHP server happening. Closer than I was a few hours ago. Still stumped as to how to access the $_POST data in PHP with any sense.

My ajax out

    var __private_api = function() {

        // workhorse.  EVERY call to our server REST API comes through
        // this interface.  NO exceptions.  We will NOT have an interface per
        // form or per MODEL as is a common case.  Makes no sense having an API really.
        // Might as well just code willy7-nilly.

        var packed = JSON.stringify(packet);

        console.log(packed);

        jqxhr = $.ajax({
            method:         "POST",                                         // ALL API calls are POST
            url:            "oop/server.php/",                              // API code
            dataType:       "json",                                         // JSON back at us please
            //contentType:    "application/json",                           // HEADER.  IMPORTANT!
            data:           {"packed":packed}                               // form data + security packet

        }).fail(function(msg) {                                             // error is depreciated.  WHY? Dunno...
            alert("Database Communication failure");                        // this is a HARD failure sent to
            console.log(JSON.stringify(msg));                               // us by the comms stack, authentication, or OS
        }).done(function(data) {                                            // AJaX worked, deal with the resultant packet
            __private_callback(data);                                       // in our custom callback. success() has been
        });                                                                 // depreciated, replaced by bone().  Why???
    }

This gets to PHP as in I can do this

   $this->database->log_config->trace(json_encode($_POST));


    $this->crud = 'EXEC';
    $SQL = "SELECT * from patients limit 2";

    switch($this->crud) {

        case 'EXEC':                                                    // before we drop into CRUDDiness, handle
            $this->database->execute($SQL);                             // well?  This eithe dies or comes back
                                                                        // it CAN come back with an empty SET
                                                                        // that is an SEP.
            $db_result = $this->database->fetch_all();                  // retrieve tuples from the CURSOR
            //$this->response['data'] = $db_result->get_stack();          // and shove in the parcel to go back to the CLIENT API
            $this->response['data'] = json_decode(json_encode($_POST),true);          // and shove in the parcel to go back to the CLIENT API
            break;

That makes it back to my client side code, and the trace from the PHP code looks like this.

{"packed":"    {\"type\":\"USER\",\"method\":\"LOGIN\",\"email\":\"root\",\"pw\":\"S0lari    s7.1\",\"remember\":\"false\"}"} 

So stuff seems to be making it over the great divide OK. HOW, in my PHP code can I test the value of 'type' to see if it is 'USER'? I have been all over encode and decode but just don't seem to be getting it!

Cheers, Mark.