Mengapa data yang tersimpan di database adalah data kosong yang di initialise?

upload-gambar.php

<?php
$ktp = $this->input->post('ktp');
$nama = $this->input->post('nama');
$gender = $this->input->post('gender');
$alamat = $this->input->post('alamat');
$foto_diri = ''; // mengapa yang ini yang tersimpan di database?
$username = $this->input->post('username');
$password = $this->input->post('password');

// main upload gambar

$config['upload_path'] = './gambar/foto_diri/';
$config['allowed_types'] = 'gif|jpg|png';
$config['encrypt_name'] = TRUE;
$this->load->library('upload', $config);
if ($this->upload->do_upload('foto_diri')){
	$foto_diri = $this->upload->data()['file_name']; // seharusnya yang ini yang tersimpan di database
}