title: Why underscores(_) is used in functions like function _save_cart in php codeigniter?
body: <p>I saw many tutorials underscores (_) are used in function.</p>
  
  <p>But I don't know why it is used and how can I use ?</p>
  
  <p>Like this : </p>
  
  <pre><code>function _save_cart()
  {
  
  }
  </code></pre>