bootstrap.php

<?php
require(__DIR__ . '/vendor/autoload.php');
require(__DIR__ . '/vendor/yiisoft/yii2/Yii.php');

$config = [
	'id' => 'micro-app',
	// the basePath of the application will be the `micro-app` directory
	'basePath' => __DIR__,
	// this is where the application will find all controllers
	// 'controllerNamespace' => 'app\controllers',
	'bootstrap' => [
		'log',
	],
	// set an alias to enable autoloading of classes from the 'app' namespace
	'aliases' => [
		'@app' => __DIR__,
	],
	'components' => [
		'db' => [
			'class' => 'yii\db\Connection',
			'dsn' => 'mysql:host=localhost;dbname=yourdatabasename',
			'username' => 'yourusername',
			'password' => 'yourpassword',
			'charset' => 'utf8',
		],
		'log' => [
			'traceLevel' => YII_DEBUG ? 3 : 0,
			'targets' => [
				[
					'class' => 'yii\log\FileTarget',
					// 'logFile' => '@app/app.log',  //if you don't want to use default (see Yii documentation)
					'levels' => ['error', 'warning'],
					'maxFileSize' => 1024,  // 1 Mb
				],
			],
		],
	],
];

if (PHP_SAPI === 'cli') {
	// when running a console application
	$application = new yii\console\Application($config);
	$exitCode = $application->run();
	exit($exitCode);
} else {
	// when running through a webserver
	(new yii\web\Application($config));
}

class Customer extends \yii\db\ActiveRecord {
	public static function tableName() {
		return 'main_customers';
	}
}