title: how can i pass second parameter as optional to the route in LUMEN?
body: <p>My routes/web.php file</p>
  
  <pre><code>$router-&gt;group(['prefix' =&gt; 'api/v1'], function () use ($router) {    
      $router-&gt;get('post/{string}/comment/{length?}', 'PostController@index');
  });
  </code></pre>
  
  <p>My controller file</p>
  
  <pre><code>public function index($string, $length = 0){
      // boy
  }
  </code></pre>
  
  <p>URLs to execute</p>
  
  <pre><code>localhost/project/public/api/v1/post/abcd/comment/1
      OR
  localhost/project/public/api/v1/post/abcd/comment
  </code></pre>
  
  <p>I want string and length values in my controller, string is not optional parameter but length is optional if i not provide it it should take 0</p>