How to call external functions from a PHP script in a WordPress plugin?

Question

I'm a WordPress beginner, and I have tried to develop plugins, but I don't know how to call external functions in a WordPress plugin. For example, I have an external PHP script called download.php in which there are several functions. To include this file, I call require_once (download.php);, but then I don't know how to call functions within download.php. For example, I want to call a function download ($filename, $time, $ban_user), where: $filename is a file path, $time is from 1 to 60 minutes, $ban_user is a user id banned. To do that, I coded

add_shortcode ('dloader', 'download_file');
function download_file ($atts){ 
    // I don't know how to code continued here //
}

Please guide me the way of coding, I am not a professional programmer.

---

add_shortcode( 'dloader', 'download_file' );

// generate shortcode [dloader filename="http://google.com"]
// the result is <a href="http://myweb.com/123456abc/">Download</a>
// where: 123456abc is the crypted parameter for the url http://google.com

function download_file( $atts, $content = null ) {
    require_once ( 'download.php' );
    // the original function: download ( $filename, $time, $ban_user ) { }
    extract( shortcode_atts( array(
        'filename' => '',
        'time'     => '',
        'ban_user' => '',
    ), $atts ) );
    if ( empty( $filename ) ) {
        return 'ERROR';
    }
    if ( empty( $time ) ) {
        $time = 60;
    }
    if ( empty( $ban_user ) ) {
        $ban_user = 0;
    }
    // value 0 is default, not banned
    $default = '<div class="dloader">';
    //execute the function download() in the file download.php
    $default .= '<a href="' .download( $filename, $time, $ban_user ). '">Download</a>';
    return $default;
}

Answer 1

Generally we use shortcodes when we have to do something in WordPress posts and pages. Though they can also be used elsewhere. Assuming that you know how to use shortcodes here is what you might want to do:-

<?php
/**
 * Plugin Name: Learning To Code WordPress
 */

defined('ABSPATH') or die("No script kiddies please!");

require_once ( dirname(__FILE__) . "/download.php");

add_shortcode ('dloader', 'download_file');
function download_file($atts) { 

    /* Default filename. */
    $default['filename'] = 'defaultFileName.txt';
    /* Default time. */
    $default['time'] = 0;
    /* Default user id. */
    $default['ban_user_id'] = 1;

    $atts = shortcode_atts( $default , $atts, 'dloader' );

    $filename = $atts['filename'];
    $time = $atts['time'];
    $ban_user_id = $atts['ban_user_id'];

    return download($filename, $time, $ban_user_id);

}

Now use the shortcode in a post or a page like this:-

[dloader filename="abc.txt" time=10 ban_user_id=1]