Yii2 (advanced app) - Creating frontend URLs in backend and vice versa

backend__config__main.php

<?php
$config = [
    'components'          => [
// [...]
        'request'            => [
            'baseUrl'             => $params['backend.baseUrl'],
        ],
        'urlManagerFrontend' => [
            'class'               => 'yii\web\UrlManager',
            'baseUrl'             => $params['frontend.baseUrl'],
            'hostInfo'            => $params['frontend.protocol'] . '://' . $params['frontend.host'],
            'enablePrettyUrl'     => true,
            'showScriptName'      => false,
            'enableStrictParsing' => false,
            'rules'               => $params['frontend.urlrules'],
        ],
        'urlManager'         => [
            'baseUrl'             => $params['backend.baseUrl'],
            'enablePrettyUrl'     => true,
            'showScriptName'      => false,
            'enableStrictParsing' => false,
            'rules'               => $params['backend.urlrules'],
        ],
// [../]
  ],
);

backend__controllers__SiteController.php

<?php
namespace backend\controllers;

class SiteController extends Controller
{
    public function actionIndex() {
        
        /**
         * Create absolute backend URL from backend app
         * 
         * @var \yii\web\UrlManager $urlManager
         */
        $urlManager = \Yii::$app->urlManager;
        echo \yii\helpers\VarDumper::dumpAsString(
            $urlManager->createAbsoluteUrl(['controller/action'])
        );
        
        /**
         * Create absolute frontend URL from backend app
         * 
         * @var \yii\web\UrlManager $urlManagerFrontend
         */
        $urlManagerFrontend = \Yii::$app->urlManagerFrontend;
        echo \yii\helpers\VarDumper::dumpAsString(
            $urlManagerFrontend->createAbsoluteUrl(['controller/action'])
        );

        // Results in 'http://example.com/admin/controller/action''http://example.com/controller/action'
    }
}

common__config__params.php

<?php
return [
// [...]
    'frontend.baseUrl'              => '',
    'frontend.urlrules'             => [
        '/' => 'site/index',
    ],
    'frontend.protocol'             => 'http',
    'frontend.host'                 => 'dev.example.com',

    'backend.baseUrl'               => '/admin',
    'backend.urlrules'              => [
        ''                                                                         => 'site/index',
        '/'                                                                        => 'site/index',
        '/login'                                                                   => 'site/login',
        '/logout'                                                                  => 'site/logout',

        '/users'                                                                   => 'users/index',
        '/users/create'                                                            => 'users/create',
        '/users/<id:\d+>'                                                          => 'users/view',
        '/users/<id:\d+>/update'                                                   => 'users/update',
        '/users/<id:\d+>/delete'                                                   => 'users/delete',
    ],
    'backend.protocol'              => 'http',
    'frontend.host'                 => 'dev.example.com',
// [...]
];

frontend__config__main.php

<?php
$config = [
    'components'          => [
// [...]
        'request'            => [
            'baseUrl'             => $params['frontend.baseUrl'],
        ],
        'urlManagerBackend' => [
            'class'               => 'yii\web\UrlManager',
            'baseUrl'             => $params['backend.baseUrl'],
            'hostInfo'            => $params['backend.protocol'] . '://' . $params['backend.host'],
            'enablePrettyUrl'     => true,
            'showScriptName'      => false,
            'enableStrictParsing' => false,
            'rules'               => $params['backend.urlrules'],
        ],
        'urlManager'         => [
            'baseUrl'             => $params['frontend.baseUrl'],
            'enablePrettyUrl'     => true,
            'showScriptName'      => false,
            'enableStrictParsing' => false,
            'rules'               => $params['frontend.urlrules'],
        ],
// [../]
  ],
);

frontend__controllers__SiteController.php

<?php
namespace frontend\controllers;

class SiteController extends Controller
{
    public function actionIndex() {
        
        /**
         * Create absolute frontend URL from frontend app
         * 
         * @var \yii\web\UrlManager $urlManager
         */
        $urlManager = \Yii::$app->urlManager;
        echo \yii\helpers\VarDumper::dumpAsString(
            $urlManager->createAbsoluteUrl(['controller/action'])
        );
        
        /**
         * Create absolute backend URL from frontend app
         * 
         * @var \yii\web\UrlManager $urlManagerBackend
         */
        $urlManagerBackend = \Yii::$app->urlManagerBackend;
        echo \yii\helpers\VarDumper::dumpAsString(
            $urlManagerBackend->createAbsoluteUrl(['controller/action'])
        );

        // Results in 'http://example.com/controller/action''http://example.com/admin/controller/action'
    }
}