namespace App\Controller;

class PostsController extends AppController {

    public function initialize(){
        parent::initialize();
        // code that you want to run before every action
    }
    public function view($id) {
       //Your code here
    }
}

The beforeFilter() method executes before any other method executes in the controller.

First use the Event namespace before defining the class in your controller file.

use Cake\Event\Event;

In your controller, add the beforeFilter() method as shown below.

public function beforeFilter(Event $event) {
    parent::beforeFilter($event);
}

Or, You can use the initialize() method.

public function initialize(){
    parent::initialize();
}

Pass each variable to view at a time

$this->set('color', 'pink');
$this->set('color', $color);

Pass multiple variables to view together via compact() function

$color1 = 'pink';
$color2 = 'red';
$this->set(compact('color1', 'color2'));

You can retrieve post data as Array.

$post_data= $this->request->data;

You can retrieve post data for particular key.

$this->request->data['field'];

Retrieve specific key value

$this->request->data('key_name');

Retrieve specific key value of nested array

$this->request->data('data.subfield');

the difference between the array notation and data() method is that data() is error safe and returns null if the key does not exist in the array

so intead of doing

if(isset($this->request->data['field']) && $this->request->data['field']) { ...}

you can do

if($this->request->data('field')) { ...}

for CakePHP 3.4.x +

get all data:

$this->request->getData();

get specific key:

$this->request->getData('key');

to set data available for getData function you have to do something like:

$this->request = $this->request->withData('some_key_on_the_fly', 'value');
$some_key_on_the_fly = $this->request->getData('some_key_on_the_fly');

useful for updating the models in controller with static data

By default CakePHP loads the related model in the controller. In order to load another model in the controller, use the loadModel() method:

$this->loadModel('Articles');

or load on the fly

$table = TableRegistry::get('Articles');
$table->find();

Redirect to within application (another action of specific controller).

return $this->redirect([
    'controller' => 'myController',
    'action' => 'myAction'
]);

Redirect to referrer page

return $this->redirect($this->referer());

Redirect to outside of application or specific URL

return $this->redirect("http://stackoverflow.com/users/1793428/haresh-vidja");

Passing Variable in URL as a method's parameter

return $this->redirect([
    'controller' => 'users',
    'action' => 'profile',
    $id
]);

Url should be looks like this http://your_app_url/users/profile/{id}

in UsersController.php file in profile() method

class UsersController extends Controller {
    public function profile($id=null) {
        $userData=$this->Users->get($id);
    }
}

Passing variable in URL as a Query String

return $this->redirect([
    'controller' => 'users',
    'action' => 'profile',
    '?'=>['id'=>$id]
]);

Url should be looks like this http://your_app_url/users/profile/?id={id}

in UsersController.php file in profile() method

class UsersController extends Controller {
    public function profile() {
        $userData=$this->Users->get($this->request->query('id'));
    }
}

Set default layout for entire application. i.e, created layout file in /src/Template/Layout/admin.ctp

class AppsController extends Controller {

    public function beforeFilter(Event $event) {
        parent::beforeFilter($event);
        $this->viewBuilder()->layout('admin'); // For Version >= 3.1 or
        $this->layout = 'admin'; // for version < 3.1
        
        // your other code should be here
    }
}

Set default layout for specific action in application. i.e, application have different layout in login page in /src/Template/Layout/login.ctp

class UsersController extends Controller {

    public function login() {

        $this->viewBuilder()->layout('login'); // For Version >= 3.1 or
        $this->layout = 'login'; // for version < 3.1
        
        //your other code should be here
    }
}

Change Layout for specific Controller. for i.e, you need different layout for all method of specific controller

class UsersController extends Controller {

    public function beforeFilter(Event $event) {
        parent::beforeFilter($event);

        $this->viewBuilder()->layout('user_layout'); // For Version >= 3.1 or
        $this->layout = 'user_layout'; // for version < 3.1
        
        //your other code should be here
    }
}

Generally in AJAX request no need of load CSS, JS. Also omitting other HTML code.

Make ajax.ctp file in /src/Template/Layout, and code should be

<?php 
    $this->fetch('content');

Set AJAX based layout for entire application, in AppsController.php

class AppsController extends Controller {

public function beforeFilter(Event $event) {
    parent::beforeFilter($event);
    if($this->request->isAjax())
    {
        $this->viewBuilder()->layout('ajax'); // For Version >= 3.1 or
        $this->layout = 'ajax'; // for version < 3.1

    }
    else
    {
        $this->viewBuilder()->layout('admin'); // For Version >= 3.1 or
        $this->layout = 'admin'; // for version < 3.1
    }
    
    // your other code should be here
}

}

We can load components in two ways.

1. By initialize or override $components property in Controller
2. By using loadComponent() method in initialize() method of Controller.

Way-1 It should be override loading component by AppsController.php load one or more component

class UsersController extends AppController {
    public $components = ['RequestHandler','Auth','Flash']; 
}

Way-2 Use this way when you need load component dynamically for specific controller. Load One component

class UsersController extends AppController {
    public function initialize() {
        parent::initialize();
        $this->loadComponent("RequestHandler"); // load specific component
        $this->loadComponent(["RequestHandler","Auth","Flash"]); // load specific component
    }
}

initialize() is introduced in CakePHP version > 3.0

As a code structure, it looks like same as beforeFilter() method. but there is many differences between beforeFilter() and initialize().

1. initialize() is always called after constructor is called. but beforeFilter() is not calling in case of action method not found in particular controller.
2. initialize() method is generally used for initialize something like add new components and helpers. but in beforeFilter() is generally used for execute some global logic part.

You can retrieve query string data as Array.

$post_data= $this->request->query;

You can retrieve post data for particular key.

$this->request->query['field'];

Retrieve specific key value

$this->request->query('key_name');

Retrieve specific key value of nested array

$this->request->query('data.subfield');

How to create User Model Class

namespace App\Model\Table;
use Cake\ORM\Table;

class UsersTable extends Table {
    public function initialize(array $config) {
        $this->table('users'); //define table name
        $this->displayField('username'); // unique or other special field of users table
        $this->primaryKey('id'); // primary key of users table
        $this->tablePrefix('prefix_'); // if prefix set tablename should be prefix_users

        // your other code here
    }

    // your other methods here
}

There are 4 types of associations(relationships) we can define in CakePHP

class PostsTable extends Table {
    public function initialize(array $config) {

        // table initialization code should be here

        $this->belongsTo('Authors', [
            'className' => 'Authors',
            'foreignKey' => 'author_id',
            'joinType' => 'INNER',
        ]);
        $this->hasMany('Tags');
        $this->hasOne('Categories');
        $this->hasAndBelongsToMany('Topics');
    }
}

In above example, you can see 4 types of relationships

Post belongs to Author (One to One), it means in posts table has one foreign key author_id which is associated with id of authors table.

Post has many Tags (One to Many), it means in tags table has one foreign key post_id which is associated with id of posts table.

Post has one Tags (Many to One or One to One), it means in posts table has one foreign key category_id which is associated with id of categories table.

Post has and belongs to Topics (Many to Many), this is many to many relationship between posts and topics table. for maintain many to many relationship must need to create third table, table, table name should be posts_categories. Fields of this table as mentioned below

1. id (primary key of table)
2. post_id (foreign key of posts table)
3. topic_id (foreign key of topics table)