title: Wordpress include php file
body: <p>I am trying to create a php website on <code>WordPress</code> for the first time.</p>
  
  <p>When I create a page, it creates a permalink which is of the form <code>http://localhost/?p=123</code>. I don't know if there is a corresponding file.</p>
  
  <p><img src="https://i.stack.imgur.com/ZhV4d.png" alt="enter image description here"></p>
  
  <p>I've installed <code>insert-php</code> plugin to read php code. It works fine on a static page. But how do I include another php file? I want to include <code>my_utilities</code> which contains all the back-end functions, in <code>login</code>. It has a permalink <code>'http://localhost/?page_id=45'</code>.</p>
  
  <p>What to do?</p>
  
  <p><code>include 'http://localhost/?page_id=45'</code> doesn't work.</p>