Problem with json_encode php and ajax

I'm using CodeIgniter where I have a driver that returns an array in json format, and I want to receive it from jquery ajax, but I get this error:

Fatal error</b>:  Call to undefined function json_encode()

It also turns out that the php version I have on the server is something old, I think it's 5.2 and maybe that's why it does not work.

public function Datos(){
    $nrofi = $this->input->post('nrofi');
    $data['datos'] = $this->Mimodel->getDatos($nrofi);
    echo json_encode($data);
}


var ficha = xx;
    $.ajax({
                        type: 'post',
                        url: "<?php echo base_url('index.php/MiController/Datos')?>",
                        data: {nrofi: ficha},
                        dataType: 'json',
                        success: function(data){
                            var d = jQuery.parseJSON(JSON.stringify(data));
                            console.log(d);

                        },
                        error: function(jqXHR, textStatus, errorThrown){
                            console.log(errorThrown);
                        }
                    });

that's my code, in case you can not use json, how can you recover the driver array from ajax ??