CakePHP 2.x supports PHP Version 5.2.8 and above while CakePHP 3.x supports PHP Version 5.4.16 & above. There are some changes from CakePHP 2.x to 3.0 like syntax declaration , array declaration & model defination etc.
The config key www_root has been changed to wwwRoot. Here is model & controller declaration examples which will give you a basic idea about CakePHP3.0

Example of usercontroller in 3.0

namespace App\Controller;
namespace App\Controller;
 class UsersController extends AppController {
 
    public $components = [
        'Paginator'
    ];
 
    public function index() {
        $this->set('users', $this->Paginator->paginate($this->Users, [
            'limit' => 5,
            'conditions' => [
                'Users.active' => 1
            ]
        ]));
    }
}

Example of usercontroller in 2.0
 

<?php
   
App::uses('AppController', 'Controller');

class AdminsController extends AppController {

    public $components = array('paginater');

        public function index() {
        $this->set('users', $this->Paginator->paginate($this->Users, array(
            'limit' => 5,
            'conditions' => array(
                'Users.active' => 1
            )
        )));
    }

}

Ex: Model In 3.0

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

class UsersTable extends Table {
 
}

//The UsersTable, formerly known as a Model in 2.0.

Ex: Model in 2.0

<?php
App::uses('AppModel', 'Model');

class User extends AppModel {

}