title: Null check on type hinting php
body: <pre><code>public function update(?User $user, Post $post)
  {
     return $user-&gt;id === $post-&gt;user_id;
  }
  </code></pre>
  
  <p>In the above function there is a '?' in front of User model type hinting.</p>
  
  <p>What is the use case of this operator in function parameter?</p>