1. What is CodeIgniter?

CodeIgniter (CI) is a free PHP based MVC framework with tons of libraries. The documentation is easy enough for a beginner to build a website. Since CI strictly follows the MVC, it is easy to separate views (templates) and business and presentation logics and thus it is convenient to work on a project with Front End and Back End programmers. CI is Faster than other frameworks (because its library consumes less memory), SEO friendly URLs (without php extension), and provides easy documentation for beginners.

Some of the important features of CodeIgniter are listed below:

Model-View-Controller (MVC) Based System
Extremely Light Weight
Full-Featured database classes with support for several platforms.
Query Builder Database Support
Form and Data Validation
Security and XSS Filtering
Session Management
Email Sending Class. Supports Attachments, HTML/Text email, multiple protocols (sendmail, SMTP, and Mail) and more.
Image Manipulation Library (cropping, resizing, rotating, etc.). Supports GD, ImageMagick, and NetPBM
File Uploading Class
FTP Class
Localization
Pagination
Data Encryption
Benchmarking
Full Page Caching
Error Logging
Calendaring Class
User Agent Class
Zip Encoding Class
Template Engine Class
Trackback Class
XML-RPC Library
Unit Testing Class
Search-engine Friendly URLs
Flexible URI Routing
Support for Hooks and Class Extensions
Large library of "helper" functions

I would like to introduce CodeIgniter framework in following sections:

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2. Installation Instructions

It is very easy to install CodeIgniter. Just follow the steps given below:
###Step-1: Download the CodeIgniter (latest version: CodeIgniter v3.1.11) from the link
###Step-2: Unzip the folder and rename it to your desired project name, for our example, we renamed it as "codeigniter-test"
###Step-3: Upload all files and folders to your server
(for windows xampp server copy the "codeigniter-test" folder to /xampp/htdocs/)
###Step-4: After uploading all the files to your server, visit the URL of your server, e.g., .
(for xampp local server, we can access our site by )

The welcome page is as follows

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3. CodeIgniter Application Architechture

As shown in the figure, whenever a request comes to CodeIgniter, it will first go to index.php page.
In the second step, Routing will decide whether to pass this request to step-3 for caching or to pass this request to step-4 for security check.
If the requested page is already in Caching, then Routing will pass the request to step-3 and the response will go back to the user.
If the requested page does not exist in Caching, then Routing will pass the requested page to step-4 for Security checks.
Before passing the request to Application Controller, the Security of the submitted data is checked. After the Security check, the Application Controller loads necessary Models, Libraries, Helpers, Plugins and Scripts and pass it on to View.
The View will render the page with available data and pass it on for Caching. As the requested page was not cached before so this time it will be cached in Caching, to process this page quickly for future requests.

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4. Directory Structure of CodeIgniter

The CodeIgniter directory structure is divided into 3 folders:
📁　Application
📁　System
📁　User_guide

###

📁Application

As the name indicates the Application folder contains all the code of your application that you are building. This is the folder where you will develop your project. The Application folder contains several other folders, which are explained below −

Cache − This folder contains all the cached pages of your application. These cached pages will increase the overall speed of accessing the pages.
Config − This folder contains various files to configure the application. With the help of config.php file, user can configure the application. Using database.php file, user can configure the database of the application.
Controllers − This folder holds the controllers of your application. It is the basic part of your application.
Core − This folder will contain base class of your application.
Helpers − In this folder, you can put helper class of your application.
Hooks − The files in this folder provide a means to tap into and modify the inner workings of the framework without hacking the core files.
Language − This folder contains language related files.
Libraries − This folder contains files of the libraries developed for your application.
Logs − This folder contains files related to the log of the system.
Models − The database login will be placed in this folder.
Third_party − In this folder, you can place any plugins, which will be used for your application.
Views − Application’s HTML files will be placed in this folder.

###

📁System

This folder contains CodeIgniter core codes, libraries, helpers and other files, which help make the coding easy. These libraries and helpers are loaded and used in web app development.

This folder contains all the CodeIgniter code of consequence, organized into various folders −
Core − This folder contains CodeIgniter’s core class. Do not modify anything here. All of your work will take place in the application folder. Even if your intent is to extend the CodeIgniter core, you have to do it with hooks, and hooks live in the application folder.
Database − The database folder contains core database drivers and other database utilities.
Fonts − The fonts folder contains font-related information and utilities.
Helpers − The helpers folder contains standard CodeIgniter helpers (such as date, cookie, and URL helpers).
Language − The language folder contains language files. You can ignore it for now.
Libraries − The libraries folder contains standard CodeIgniter libraries (to help you with e-mail, calendars, file uploads, and more). You can create your own libraries or extend (and even replace) standard ones, but those will be saved in the application/libraries directory to keep them separate from the standard CodeIgniter libraries saved in this particular folder.

###

📁User_guide

This is your user guide to CodeIgniter. It is basically, the offline version of user guide on the CodeIgniter website. Using this, one can learn the functions of various libraries, helpers and classes. It is recommended to go through this user guide before building your first web app in CodeIgniter.

Besides these three folders, there is one more important file named index.php. In this file, we can set the application environment and error level and we can define system and application folder name. It is recommended, not to edit these settings if you do not have enough knowledge about what you are going to do.

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5. Create a new view page

The default controller is Welcome controller which is located at application\controllers\Welcome.php.

###

Step 1: Create a Controller

Now, for a test we will create another Controller named Sample (application\controllers\Sample.php) as follows:

PHP

<?php
class Sample extends CI_Controller {

  public function index(){
    echo "Hello World!";
  }

  public function hello(){
    echo "This is hello function.";
  }

  public function test(){
    $this->load->view('test');
  }

}
?>

###

Step 2: Create a view page (template)

Now, we will create a view page named Test.php (application\views\test.php) that the Sample controller will call like following:

test.php

<!DOCTYPE html>
<html lang="en">

<head>
 <meta charset="utf-8">
 <title>CodeIgniter View Example</title>
</head>

<body>
 CodeIgniter View Example
</body>

</html>

###

Step 3: Access page by hitting siteurl/index.php/controller/method/

For the above example, Controller name is Sample, the method name is test, so, our URL will be like, https://codeigniter-test.localhost/index.php/sample/test

So, If we hit the https://codeigniter-test.localhost/index.php/sample/test then we will see the following page

###

Step 4: We can clean the URL by following methods, that is we can remove index.php from the URL.

For removing index.php from the URL https://codeigniter-test.localhost/index.php/sample/test →https://codeigniter-test.localhost/sample/test
####(a). Replace the following config setting from application\config\config.php
$config['index_page'] = 'index.php';　→　$config['index_page'] = '';　and
make sure the $config['uri_protocol'] = 'REQUEST_URI'; or $config['uri_protocol'] = 'PATH_INFO';

config.php

<?php

$config['index_page'] = '';

$config['uri_protocol']	= 'REQUEST_URI';
?>

####(b). Place the .htaccess in the root folder of your project (codeigniter-test)

.htaccess

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteBase /
  RewriteRule ^index\.php$ - [L]
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule . /index.php [L]
</IfModule>

Now, if we hit by https://codeigniter-test.localhost/sample/ that contains only the controller name, meaning to load the index method of Sample Controller. Since we put echo "Hello World!"; in the index method, the following page will be displayed:

If we hit by https://codeigniter-test.localhost/sample/test the following screen will appear

####(c). Add a suffix in URLs
If we want a add suffix .html in URLs in CodeIgniter its very easy.

Replace the following config setting from application\config\config.php
$config['url_suffix'] = '';　→　$config['url_suffix'] = '.html';

config.php

<?php

$config['url_suffix'] = '.html';

?>

Now, we can access pages by .html suffix, that is, we can access the test page by https://codeigniter-test.localhost/sample/test.html and https://codeigniter-test.localhost/sample/test.