Using Yii in third party apps

Legacy code. It happened to all of us. It's hard but we still need to deal with it. Would it be cool to gradually move from legacy towards Yii? Absolutely.

In this recipe you'll learn how to use Yii features in an existing PHP application.

How to do it

First of all, we need Yii itself. Since existing legacy application already takes care about routing, we don't need any application template. Let's start with composer.json. Either use existing one or create it:

    "name": "mysoft/mylegacyapp",
    "description": "Legacy app",
    "keywords": ["yii2", "legacy"],
    "homepage": "",
    "type": "project",
    "license": "Copyrighted",
    "minimum-stability": "dev",
    "require": {
        "php": ">=5.4.0",
        "yiisoft/yii2": "*"
    "config": {
        "process-timeout": 1800
    "extra": {
        "asset-installer-paths": {
            "npm-asset-library": "vendor/npm",
            "bower-asset-library": "vendor/bower"

Now run composer install and you should get Yii in vendor directory.

Note: The dir should not be accessible from the web. Either keep it out of webroot or deny directory access.

Now create a file that will initialize Yii. Let's call it yii_init.php:

// set it to false when in production
defined('YII_DEBUG') or define('YII_DEBUG', true);

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

$config = require(__DIR__ . '/config/yii.php');

new yii\web\Application($config);

Now create a config config/yii.php:


return [
    'id' => 'myapp',
    'basePath' => dirname(__DIR__),
    'bootstrap' => ['log'],
    'components' => [
        'request' => [
            // !!! insert a secret key in the following (if it is empty) - this is required by cookie validation
            'cookieValidationKey' => '',
        'cache' => [
            'class' => 'yii\caching\FileCache',
        'user' => [
            'identityClass' => 'app\models\User',
            'enableAutoLogin' => true,
        'log' => [
            'traceLevel' => YII_DEBUG ? 3 : 0,
            'targets' => [
                    'class' => 'yii\log\FileTarget',
                    'levels' => ['error', 'warning'],
    'params' => [],

That's it. You can require the file and mix Yii code into your legacy app:

// legacy code

$id = (int)$_GET['id'];

// new code
require 'path/to/yii_init.php';

$post = \app\models\Post::find()->where(['id' => $id])->one();

echo Html::encode($post->title);

How it works

In the yii_init.php we are including framework classes and composer autoloading. Important part there is that ->run() method isn't called like it's done in normal Yii application. That means that we're skipping routing, running controller etc. Legacy app already doing it.