haoshengxin/vendor/yiisoft/yii2/web/migrations/m160313_153426_session_init.php

<?php
/**
 * @link http://www.yiiframework.com/
 * @copyright Copyright (c) 2008 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */

use yii\db\Migration;

/**
 * Initializes Session tables
 *
 * @author Misbahul D Munir <misbahuldmunir@gmail.com>
 * @since 2.0.8
 */
class m160313_153426_session_init extends Migration
{

    /**
     * @inheritdoc
     */
    public function up()
    {
        $tableOptions = null;
        if ($this->db->driverName === 'mysql') {
            // http://stackoverflow.com/questions/766809/whats-the-difference-between-utf8-general-ci-and-utf8-unicode-ci
            $tableOptions = 'CHARACTER SET utf8 COLLATE utf8_unicode_ci ENGINE=InnoDB';
        }

        $this->createTable('{{%session}}', [
            'id' => $this->string()->notNull(),
            'expire' => $this->integer(),
            'data' => $this->binary(),
            'PRIMARY KEY ([[id]])',
        ], $tableOptions);
    }

    /**
     * @inheritdoc
     */
    public function down()
    {
        $this->dropTable('{{%session}}');
    }
}