Complete CodeIgniter Framework Cheat Sheet: Developer’s Reference Guide

Introduction to CodeIgniter

CodeIgniter is a lightweight, open-source PHP framework built for developers who need a simple, elegant toolkit to create full-featured web applications. Known for its small footprint, exceptional performance, and minimal configuration requirements, CodeIgniter follows an MVC (Model-View-Controller) architectural pattern. It provides a rich set of libraries for common tasks while maintaining flexibility, allowing developers to focus on building applications rather than framework intricacies.

Core Concepts

MVC Architecture in CodeIgniter

Model: Handles data and business logic
View: Presents data to users (UI/UX)
Controller: Processes requests, interacts with models, and loads views

Directory Structure

application/
  ├── config/          # Configuration files
  ├── controllers/     # Controller classes
  ├── core/            # Core system extensions
  ├── helpers/         # Helper functions
  ├── hooks/           # Hook classes
  ├── language/        # Language files
  ├── libraries/       # Custom libraries
  ├── models/          # Model classes
  ├── third_party/     # Third-party packages
  └── views/           # View files
system/                # CodeIgniter core files
public/                # Web-accessible files
  ├── index.php        # Front controller
  ├── assets/          # CSS, JS, images
  └── .htaccess        # URL rewriting rules

Installation and Configuration

Installation Methods

Manual Installation:
- Download from codeigniter.com
- Extract files to web server directory
- Configure application/config/config.php

Composer Installation (CI4):

composer create-project codeigniter4/appstarter project-name

Essential Configuration Files

File	Purpose	Key Settings
config.php	Core configuration	Base URL, encryption key, session settings
database.php	Database connections	DB credentials, connection settings
routes.php	URL routing rules	Default controller, custom routes
autoload.php	Auto-loaded resources	Models, libraries, helpers to load automatically

Environment Setup (CI4)

// .env file
CI_ENVIRONMENT = development  # or production
app.baseURL = 'http://localhost:8080/'
database.default.hostname = localhost
database.default.database = ci4_database
database.default.username = root
database.default.password = password

Controllers

Basic Controller

<?php
// application/controllers/Blog.php
class Blog extends CI_Controller {
    public function index() {
        $this->load->view('blog_view');
    }
    
    public function post($id) {
        $data['post_id'] = $id;
        $this->load->view('post_view', $data);
    }
}

Loading Resources

$this->load->model('blog_model');         // Load model
$this->load->library('session');          // Load library
$this->load->helper('url');               // Load helper
$this->load->database();                  // Load database
$this->load->view('view_name', $data);    // Load view with data

Returning Different Response Types

// JSON Response
public function get_data() {
    $data = $this->some_model->get_data();
    $this->output
        ->set_content_type('application/json')
        ->set_output(json_encode($data));
}

// XML Response
public function get_xml() {
    $this->load->helper('xml');
    $xml = array_to_xml($data, '<root/>');
    $this->output
        ->set_content_type('text/xml')
        ->set_output($xml->asXML());
}

Models

Basic Model

<?php
// application/models/Blog_model.php
class Blog_model extends CI_Model {
    public function __construct() {
        parent::__construct();
        $this->load->database();
    }
    
    public function get_posts($limit = 10, $offset = 0) {
        $query = $this->db->get('posts', $limit, $offset);
        return $query->result();
    }
    
    public function get_post($id) {
        $query = $this->db->get_where('posts', ['id' => $id]);
        return $query->row();
    }
}

Database Operations

Query Builder

// SELECT
$query = $this->db->get('table_name');              // SELECT * FROM table_name
$query = $this->db->get_where('table', ['id' => 1]); // SELECT * FROM table WHERE id = 1

// Customizing Queries
$this->db->select('title, content, date');
$this->db->from('posts');
$this->db->where('status', 'published');
$this->db->order_by('date', 'DESC');
$this->db->limit(10);
$query = $this->db->get();

// INSERT
$data = [
    'title' => 'New Post',
    'content' => 'Post content',
    'author_id' => 1
];
$this->db->insert('posts', $data);
$insert_id = $this->db->insert_id();

// UPDATE
$data = ['title' => 'Updated Title'];
$this->db->where('id', 5);
$this->db->update('posts', $data);

// DELETE
$this->db->delete('posts', ['id' => 5]);

// JOIN
$this->db->select('posts.*, categories.name as category');
$this->db->from('posts');
$this->db->join('categories', 'categories.id = posts.category_id');
$query = $this->db->get();

// Raw Queries
$query = $this->db->query("SELECT * FROM posts WHERE author_id = ?", [3]);

Transactions

$this->db->trans_begin();

$this->db->query('INSERT INTO users (username) VALUES ("user1")');
$this->db->query('INSERT INTO profiles (user_id) VALUES (?)', [$user_id]);

if ($this->db->trans_status() === FALSE) {
    $this->db->trans_rollback();
    return false;
} else {
    $this->db->trans_commit();
    return true;
}

Views

Basic View

<!-- application/views/blog_view.php -->
<!DOCTYPE html>
<html>
<head>
    <title><?= $title ?? 'My Blog' ?></title>
</head>
<body>
    <h1><?= $heading ?? 'Welcome to My Blog' ?></h1>
    <div class="content">
        <?php foreach ($posts as $post): ?>
            <article>
                <h2><?= $post->title ?></h2>
                <p><?= $post->excerpt ?></p>
            </article>
        <?php endforeach; ?>
    </div>
</body>
</html>

View Partials

// Loading a partial view
$this->load->view('header', $header_data);
$this->load->view('content', $content_data);
$this->load->view('footer', $footer_data);

// Returning view as string instead of outputting
$string = $this->load->view('view_name', $data, TRUE);

Routing

Basic Routes

// application/config/routes.php
$route['default_controller'] = 'home';
$route['404_override'] = 'errors/page_404';
$route['translate_uri_dashes'] = FALSE;

// Static routes
$route['about'] = 'pages/about';
$route['contact'] = 'pages/contact';

// Dynamic routes with parameters
$route['blog/(:num)'] = 'blog/post/$1';
$route['products/(:any)'] = 'catalog/product_details/$1';

// RESTful routes (CI4)
$route->resource('api/users');  // Creates all REST endpoints for users

Route Groups (CI4)

$routes->group('admin', function($routes) {
    $routes->add('users', 'Admin\Users::index');
    $routes->add('products', 'Admin\Products::index');
});
// Creates: /admin/users and /admin/products

Form Handling

Form Creation

<?= form_open('controller/method', ['class' => 'form']); ?>
    <div>
        <?= form_label('Username:', 'username'); ?>
        <?= form_input(['name' => 'username', 'id' => 'username', 'value' => set_value('username')]); ?>
        <?= form_error('username'); ?>
    </div>
    <div>
        <?= form_label('Email:', 'email'); ?>
        <?= form_input(['name' => 'email', 'id' => 'email', 'type' => 'email', 'value' => set_value('email')]); ?>
        <?= form_error('email'); ?>
    </div>
    <div>
        <?= form_submit('submit', 'Submit', ['class' => 'btn']); ?>
    </div>
<?= form_close(); ?>

Form Validation

// Controller
public function register() {
    $this->load->library('form_validation');
    
    $this->form_validation->set_rules('username', 'Username', 'required|min_length[5]|is_unique[users.username]');
    $this->form_validation->set_rules('email', 'Email', 'required|valid_email|is_unique[users.email]');
    $this->form_validation->set_rules('password', 'Password', 'required|min_length[8]');
    $this->form_validation->set_rules('passconf', 'Password Confirmation', 'required|matches[password]');
    
    if ($this->form_validation->run() == FALSE) {
        $this->load->view('register_form');
    } else {
        $this->user_model->register_user();
        $this->load->view('register_success');
    }
}

Sessions

Session Management

// Initialize
$this->load->library('session');

// Set session data
$this->session->set_userdata('name', 'John Doe');
$this->session->set_userdata(['email' => 'john@example.com', 'logged_in' => TRUE]);

// Get session data
$name = $this->session->userdata('name');
$all_data = $this->session->userdata();

// Check if session data exists
if ($this->session->has_userdata('logged_in')) {
    // User is logged in
}

// Remove session data
$this->session->unset_userdata('name');
$this->session->unset_userdata(['email', 'logged_in']);

// Set flash data (available for the next request only)
$this->session->set_flashdata('message', 'Registration successful!');

// Get flash data
$message = $this->session->flashdata('message');

// Destroy session
$this->session->sess_destroy();

Helper Functions

URL Helper

// Load the helper
$this->load->helper('url');

// Generate links
echo base_url();                        // http://example.com/
echo site_url('blog/post/123');         // http://example.com/index.php/blog/post/123
echo anchor('blog/post/123', 'Read More'); // <a href="http://example.com/index.php/blog/post/123">Read More</a>

// Redirects
redirect('controller/method');          // Redirects browser

Form Helper

$this->load->helper('form');

echo form_open('email/send');               // <form action="http://example.com/index.php/email/send" method="post">
echo form_input('username', 'johndoe');     // <input type="text" name="username" value="johndoe" />
echo form_password('password', '');         // <input type="password" name="password" value="" />
echo form_submit('submit', 'Submit');       // <input type="submit" name="submit" value="Submit" />
echo form_close();                          // </form>

Array Helper

$this->load->helper('array');

$person = ['name' => 'John', 'email' => 'john@example.com', 'status' => 'active'];
$name = element('name', $person, 'Unknown');  // Returns 'John' or 'Unknown' if not found

Text Helper

$this->load->helper('text');

$string = 'This is a long paragraph that needs to be truncated...';
echo character_limiter($string, 20);     // This is a long...
echo word_limiter($string, 4);           // This is a long...
echo word_censor($string, ['long']);     // This is a **** paragraph...

Common Libraries

Email Library

$this->load->library('email');

$config['protocol'] = 'smtp';
$config['smtp_host'] = 'smtp.example.com';
$config['smtp_user'] = 'email@example.com';
$config['smtp_pass'] = 'password';
$config['smtp_port'] = 465;
$config['smtp_crypto'] = 'ssl';
$config['mailtype'] = 'html';

$this->email->initialize($config);

$this->email->from('sender@example.com', 'Sender Name');
$this->email->to('recipient@example.com');
$this->email->cc('cc@example.com');
$this->email->bcc('bcc@example.com');
$this->email->subject('Email Subject');
$this->email->message('Email body content');
$this->email->attach('/path/to/file.pdf');

if ($this->email->send()) {
    echo 'Email sent successfully';
} else {
    echo $this->email->print_debugger();
}

Upload Library

$this->load->library('upload');

$config['upload_path'] = './uploads/';
$config['allowed_types'] = 'gif|jpg|png|pdf';
$config['max_size'] = 2048; // KB
$config['encrypt_name'] = TRUE;

$this->upload->initialize($config);

if (!$this->upload->do_upload('userfile')) {
    $error = $this->upload->display_errors();
    // Handle error
} else {
    $data = $this->upload->data();
    // File uploaded successfully
    // $data['file_name'] contains the new filename
}

Image Manipulation Library

$this->load->library('image_lib');

// Resize image
$config['image_library'] = 'gd2';
$config['source_image'] = './uploads/original.jpg';
$config['create_thumb'] = TRUE;
$config['maintain_ratio'] = TRUE;
$config['width'] = 800;
$config['height'] = 600;

$this->image_lib->initialize($config);

if (!$this->image_lib->resize()) {
    echo $this->image_lib->display_errors();
}

// Crop image
$this->image_lib->clear();
$config['image_library'] = 'gd2';
$config['source_image'] = './uploads/original.jpg';
$config['width'] = 300;
$config['height'] = 300;
$config['x_axis'] = 100;
$config['y_axis'] = 100;

$this->image_lib->initialize($config);

if (!$this->image_lib->crop()) {
    echo $this->image_lib->display_errors();
}

Pagination Library

$this->load->library('pagination');

$config['base_url'] = site_url('blog/index');
$config['total_rows'] = $this->blog_model->count_all();
$config['per_page'] = 10;
$config['uri_segment'] = 3;
$config['full_tag_open'] = '<ul class="pagination">';
$config['full_tag_close'] = '</ul>';
$config['first_link'] = 'First';
$config['last_link'] = 'Last';
$config['first_tag_open'] = '<li>';
$config['first_tag_close'] = '</li>';
$config['prev_link'] = '&laquo';
$config['prev_tag_open'] = '<li class="prev">';
$config['prev_tag_close'] = '</li>';
$config['next_link'] = '&raquo';
$config['next_tag_open'] = '<li>';
$config['next_tag_close'] = '</li>';
$config['last_tag_open'] = '<li>';
$config['last_tag_close'] = '</li>';
$config['cur_tag_open'] = '<li class="active"><a href="#">';
$config['cur_tag_close'] = '</a></li>';
$config['num_tag_open'] = '<li>';
$config['num_tag_close'] = '</li>';

$this->pagination->initialize($config);

$page = ($this->uri->segment(3)) ? $this->uri->segment(3) : 0;
$data['results'] = $this->blog_model->get_posts($config['per_page'], $page);
$data['links'] = $this->pagination->create_links();

$this->load->view('blog_view', $data);

Security Features

Cross-Site Request Forgery (CSRF) Protection

// Enable CSRF protection in config/config.php
$config['csrf_protection'] = TRUE;
$config['csrf_token_name'] = 'csrf_token';
$config['csrf_cookie_name'] = 'csrf_cookie';
$config['csrf_expire'] = 7200;
$config['csrf_regenerate'] = TRUE;

// In your view form
<?= form_open('controller/method'); ?>
<!-- CSRF token is automatically included -->

XSS Filtering

// In config/config.php
$config['global_xss_filtering'] = TRUE;  // Global XSS filtering

// Manual filtering
$clean_data = $this->security->xss_clean($input_data);

SQL Injection Prevention

// Query binding (preferred method)
$this->db->query("SELECT * FROM users WHERE username = ?", [$username]);

// Using Query Builder (auto-escaped)
$this->db->get_where('users', ['username' => $username]);

Custom Libraries

Creating a Custom Library

<?php
// application/libraries/Pdf.php
class Pdf {
    public function __construct() {
        // Constructor code
        log_message('debug', 'PDF Library Initialized');
    }
    
    public function generate($html, $filename) {
        // Library methods
    }
}

// Using the library
$this->load->library('pdf');
$this->pdf->generate($html, 'document.pdf');

Extending Native Libraries

<?php
// application/libraries/MY_Email.php
class MY_Email extends CI_Email {
    public function __construct() {
        parent::__construct();
    }
    
    public function quick_send($to, $subject, $message) {
        $this->from('system@example.com', 'System');
        $this->to($to);
        $this->subject($subject);
        $this->message($message);
        return $this->send();
    }
}

// Using the extended library
$this->load->library('email');
$this->email->quick_send('user@example.com', 'Test', 'Test message');

Hooks

Hook Points and Configuration

// application/config/hooks.php
$hook['pre_system'] = [
    'class'    => 'MyHook',
    'function' => 'pre_system_method',
    'filename' => 'MyHook.php',
    'filepath' => 'hooks'
];

$hook['post_controller_constructor'] = [
    'class'    => 'Auth',
    'function' => 'check_access',
    'filename' => 'Auth.php',
    'filepath' => 'hooks'
];

// Enable hooks in config/config.php
$config['enable_hooks'] = TRUE;

Hook Class Example

<?php
// application/hooks/MyHook.php
class MyHook {
    public function pre_system_method() {
        // Execute before the system execution
    }
}

Common Challenges & Solutions

Problem: “404 Page Not Found” Errors

Check if the controller class name starts with uppercase letter
Verify that the file name matches the class name exactly
Make sure .htaccess is properly configured for URL rewriting
Check if routes are properly defined in routes.php

Problem: Database Connection Issues

Double-check database credentials in database.php
Verify that database server is running
Check for proper database driver installation
Try connecting with a standalone PHP script to isolate the issue

Problem: Session Data Not Persisting

Check session configuration in config.php
Verify session storage directory is writable
Make sure session cookies are not being blocked
Check for session timeout settings

Problem: File Upload Errors

Verify upload directory permissions (chmod 777 for testing)
Check php.ini settings for upload_max_filesize and post_max_size
Ensure proper enctype=”multipart/form-data” in form tag
Check for file type restrictions in upload configuration

Best Practices

Follow CodeIgniter’s style guide (found in user guide)
Use Query Builder instead of raw SQL when possible
Implement proper input validation for all user inputs
Organize business logic in models, not controllers
Use libraries for reusable code across multiple controllers
Leverage helpers for simple utility functions
Implement proper error handling and logging
Use environment-specific configuration files
Create base controllers for common functionality
Document your code thoroughly with PHPDoc comments

CodeIgniter 4 Specific Features

Namespaces

namespace App\Controllers;

use App\Models\UserModel;
use CodeIgniter\Controller;

class Users extends Controller {
    public function index() {
        $model = new UserModel();
        $data['users'] = $model->findAll();
        return view('users/index', $data);
    }
}

Entity Classes

namespace App\Entities;

use CodeIgniter\Entity\Entity;

class User extends Entity {
    protected $casts = [
        'id' => 'integer',
        'active' => 'boolean',
        'created_at' => 'datetime',
    ];
    
    public function setPassword(string $password) {
        $this->attributes['password'] = password_hash($password, PASSWORD_BCRYPT);
        return $this;
    }
    
    public function verifyPassword(string $password) {
        return password_verify($password, $this->attributes['password']);
    }
}

Filters (Middleware)

namespace App\Filters;

use CodeIgniter\HTTP\RequestInterface;
use CodeIgniter\HTTP\ResponseInterface;
use CodeIgniter\Filters\FilterInterface;

class Auth implements FilterInterface {
    public function before(RequestInterface $request, $arguments = null) {
        if (!session()->get('logged_in')) {
            return redirect()->to('/login');
        }
    }
    
    public function after(RequestInterface $request, ResponseInterface $response, $arguments = null) {
        // Do something after the controller execution
    }
}

// app/Config/Filters.php
public $filters = [
    'auth' => [
        'before' => [
            'admin/*',
            'dashboard',
        ]
    ]
];

Command-Line Interface

# Create a new controller
php spark make:controller Blog

# Create a new model
php spark make:model UserModel

# Create a migration
php spark make:migration create_users_table

# Run migrations
php spark migrate

# Rollback migration
php spark migrate:rollback

# Seed the database
php spark db:seed UserSeeder

# Show routes
php spark routes

# Run the development server
php spark serve

Resources for Further Learning

Official Documentation

Community Resources

Recommended Books

“Pro PHP and jQuery with CodeIgniter” by Brad Dayley
“CodeIgniter Web Application Blueprints” by Rob Foster
“Learning CodeIgniter” by Sk Arshad

Video Tutorials

Codecourse – CodeIgniter Tutorial Series
YouTube channel: “ion_auth”
Udemy: “Learn CodeIgniter 4 From Scratch”

This cheat sheet covers the essential aspects of CodeIgniter for both versions 3 and 4, with specific CI4 features highlighted separately. It should serve as a comprehensive reference for both beginners and intermediate developers working with this framework.