title: preg_replace all spaces
body: <p>I'm trying to replace all spaces with underscores and the following is not working:</p>
  
  <pre><code>$id = "aa aa";
  echo $id;
  preg_replace('/\s+/', '_', $id);
  echo $id;
  </code></pre>
  
  <p>prints</p>
  
  <pre><code>aa aaaa aa
  </code></pre>