How can I delete the first quotes and the last of a string in php

I need to delete the first and last quotes of a string which come from android Studio with Json format but send this Json as a string of characters or a Json Object here I leave the code in android

  private void insertar() {

        RequestQueue queue = Volley.newRequestQueue(getActivity());
        final JSONObject datos = new JSONObject();

        try {
            datos.put("idnummodelo", "25469" );
            datos.put("idsubnummodelo", "78");
            datos.put("idestado", "2");
            datos.put("idtipodemodelo", "4");
            datos.put("idclasificaciondeseguridad", "1");
            datos.put("idpiedefirma", "56");
            datos.put("nombredescriptivo", "Android new");
            datos.put("periodicidad", "2");
            datos.put("aproximacion", "4");
            datos.put("idsistemadeinformacion", "ONEI");
            datos.put("notametodologica", "blah");
            datos.put("denominaciondelpie", "Sistema");
            datos.put("primercargo", "blah");
            datos.put("segundocargo", "blah");
            datos.put("notadecertificacion", "blah");
        } catch (JSONException e) {
            // handle exception
            Toast.makeText(getActivity(), "JSONException", Toast.LENGTH_SHORT).show();

        }
        Toast.makeText(getActivity(), datos.toString(), Toast.LENGTH_LONG).show();
        JsonObjectRequest putRequest = new JsonObjectRequest(Request.Method.POST, "http://" + JSON_ARRAY_REQUEST_IP + "/api/vista/plantilla.php?accion=InsertarEncuesta", datos,
                new Response.Listener<JSONObject>()
                {
                    @Override
                    public void onResponse(JSONObject response) {
                        // response
                        Toast.makeText(getActivity(), response.toString(), Toast.LENGTH_SHORT).show();
                    }
                },
                new Response.ErrorListener()
                {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        // error
                        Toast.makeText(getActivity(), error.toString(), Toast.LENGTH_SHORT).show();
                    }
                }
        ) {

            @Override
            public Map<String, String> getHeaders()
            {
                Map<String, String> headers = new HashMap<String, String>();
                headers.put("Content-Type", "application/json");
                return headers;
            }

            @Override
            public byte[] getBody() {

                try {
                    Log.i("json", datos.toString());
                    return datos.toString().getBytes("UTF-8");
                } catch (UnsupportedEncodingException e) {
                    e.printStackTrace();
                }
                return null;
            }
        };

        queue.add(putRequest);
    }

This returns this result when I print with

Toast.makeText(getActivity(), response.toString(), Toast.LENGTH_SHORT).show();

datos = "{"idnummodelo":"25469","idsubnummodelo":"78", "idestado":"2","idtipodemodelo":"4","idclasificaciondeseguridad":"1","idpiedefirma":"56", "nombredescriptivo":"Android new","periodicidad":"2","aproximacion":"4","idsistemadeinformacion":"ONEI","notametodologica":"blah","denominaciondelpie":"Sistema","primercargo":"blah","segundocargo":"blah","notadecertificacion":"blah"}"

I would like to delete the quotes to get a Json so I can work on it like this:

$Ariel = ($_POST['datos']);

$obj=  json_decode($Ariel);


include '../modelo/dao-api.php';
$ob = new daoApi();


$result=$ob->InsertarEncuesta($obj);


header("Access-Control-Allow-Origin: *");
header('Content-Type: application/json'); 

echo json_encode('Esto inserto correctamente');