CodeIgniter 3.x Migrations example

<?php
defined('BASEPATH') OR exit('Forbidden');

/**
 * Class Migration_create_users_table
 * @property CI_DB_forge $Forge
 */
class Migration_create_users_table extends CI_Migration
{
    public function up()
    {
        $this->Forge
            ->add_field([
                [
                    'username' => [
                        'type' => 'VARCHAR',
                        'constraint' => 30
                    ],
                    'password' => [
                        'type' => 'VARCHAR',
                        'constraint' => 255
                    ],
                    'active' => [
                        'type' => 'BOOLEAN',
                        'default' => 0
                    ]
                ]
            ])
            ->add_key('id');

        $this->Forge
            ->create_table('table', true);
    }
}