Calling php function from inside another function

Maybe you guys can help me I am trying to do this:

public function index()
{
    $r = array();
    //some code   
    echo json_encode($this->utf8ize($r));
}

public function utf8ize($d) {
   //some code
    return $d;
}

But I get the "Call to undefined function utf8ize()" error

Why?

Edit 1: The complete code

<?php
defined('BASEPATH') OR exit('No direct script access allowed');
class Solicitud extends MX_Controller {

public function __construct()
{
    /*
    parent::__construct();

    if(!$this->input->is_ajax_request())
    {
        show_404();
        exit();
    }
    else
    {
    */
        $this->load->model('Solicitud_model', 'Model');
    //}
}

public function index()
{
    $bandera = $this->input->post('bandera');
    $r = array();
    if ($bandera == 1){
        $result = $this->Model->getConsulta($this->session->session_facultad_apps);
        $r = array("data"    => $result,
            "success" => true,
            "bandera" => $bandera);
    }else if($bandera == 2)
    {
        $result = $this->Model->get($this->session->session_facultad_apps);
        $r = array("data"    => $result,
            "success" => true,
            "bandera" => $bandera);

    }else if ($bandera == 3){
        $result = $this->Model->getAsigna($this->session->session_facultad_apps);
        $r = array("data"    => $result,
            "success" => true,
            "bandera" => $bandera);
    }


    echo json_encode(utf8ize($r));
}

public function utf8ize($d) {
    if (is_array($d)) {
        foreach ($d as $k => $v) {
            $d[$k] = utf8ize($v);
        }
    } else if (is_string ($d)) {
        $d = iconv('UTF-8', 'ISO-8859-1', $d);
        return utf8_encode($d);
    }
    return $d;
}