$config = [
  'modules' => [
    'comment' => [
      'class' => 'frontend\modules\comment\Comment',
    ],
    'article' => [
      'class' => 'frontend\modules\article\Article',
      'modules' => [
        'category' => [
          'class' => 'frontend\modules\article\modules\category\Category',
        ],
        'title' => [
            'class' => 'frontend\modules\article\modules\title\Title',
        ],
      ],
    ],
  ],
];
if (!YII_ENV_TEST) {
  // configuration adjustments for 'dev' environment
  $config['bootstrap'][] = 'debug';
  $config['modules']['debug'] = [
    'class' => 'yii\debug\Module',
  ];
  $config['bootstrap'][] = 'gii';
  $config['modules']['gii'] = [
    'class' => 'yii\gii\Module',
  ];
}
return $config;

<?php
namespace frontend\modules\article;
/**
 * article module definition class
 */
class Article extends \yii\base\Module
{
  /**
   * @inheritdoc
   */
  public $controllerNamespace = 'frontend\modules\article\controllers';
  /**
   * @inheritdoc
   */
  public function init()
  {
    parent::init();
    //添加子模塊后
    $this->modules = [
      'category' => [
        'class' => 'frontend\modules\article\modules\category\Category',
      ],
    ];
    // custom initialization code goes here
  }
}