title: WordPress add a new page to admin section
body: <p>I have already developed my plugin for WordPress and I can manage it from admin. I have passed the access to the plugin file using <code>add_submenu_page</code>. The problem is that the plugin is extending and I want to use another file that is linked from the main file. For example I have <code>second_page.php?id=3</code>. When I try to access this link, I get a </p>
  
  <blockquote>
    <p>You do not have sufficient permissions to access this page.</p>
  </blockquote>
  
  <p>message. I want to "validate" this page also for using with this script and I don't know how. Ideas?</p>