title: PHP string interpolation syntax
body: <p>I tried to do redirect with this syntax:</p>
  
  <pre><code>header("location: readMore.php?id=$post['post_id']");
  </code></pre>
  
  <p>But it didn't work. It worked only after someone suggested to put curly brackets around <code>$post['post_id']</code>!</p>
  
  <p>The correct syntax is:</p>
  
  <pre><code>header("location: readMore.php?id={$post['post_id']}");
  </code></pre>
  
  <p>What does the curly brackets do in this case?</p>