config/main.php

  'urlManagerBackend' => [
            'class' => 'yii\web\urlManager',
            'baseUrl' => '/yii2-app-advanced/backend/web/',
            'enablePrettyUrl' => true,
            'showScriptName' => false,
        ],

views/layouts/main.php

 $menuItems = [  ...
        ['label'=>'backend', 'url'=>Yii::$app->urlManagerBackend->baseUrl],
        ...
    ];