How can I save my uploaded file's name in a variable (I'm new to CI)?

I am using this code for testing:

I would really appreciate your help on this guys.

This is the code I am using (and I am typing this over and over again because of the restrictions of stackoverflow):

<?php

class Gallery_model extends CI_Model {

    var $gallery_path;
    var $videos_path;
    var $thumbnail;
    var $video_name;

    function Gallery_model() {

        parent::__construct();

        $this->gallery_path = realpath(APPPATH . '../images');
        $this->videos_path = realpath(APPPATH . '../videos');
        $this->videos_path_tnumb = realpath(APPPATH . '../videos/thumb');

        $this->thumbnail = 'C://xampp//htdocs//upload//videos//thumb';
        $this->video_name = 'C://xampp//htdocs//upload//videos//79825_00_01_SC03_intro.mov';
    }

    function do_upload() {

        $config = array(
//            'allowed_types' => 'jpg|jpeg|png|gif',
//            'upload_path' => $this->gallery_path,
//            'max_size' => 10000

            'allowed_types' => 'avi|mp4|flw|mov',
            'upload_path' => $this->videos_path
        );

        $this->load->library('upload', $config);
        $field_name = "some_field_name";
        if ($this->upload->do_upload()) {
            $this->getThumbImage($_POST['userfile']);
        }
    }

    function getVideoInformation($videoPath) {
        $movie = new ffmpeg_movie($videoPath, false);

        $this->videoDuration = $movie->getDuration();
        $this->frameCount = $movie->getFrameCount();
        $this->frameRate = $movie->getFrameRate();
        $this->videoTitle = $movie->getTitle();
        $this->author = $movie->getAuthor();
        $this->copyright = $movie->getCopyright();
        $this->frameHeight = $movie->getFrameHeight();
        $this->frameWidth = $movie->getFrameWidth
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