Example of denying a page load based on Drupal 7 permissions

permission_denied.php

<?php
/**
 * Implements hook_init().
 *
 * Runs every time a page is requested on the site.
 */
function my_custom_module_init() {
  // Return a 403 page and prevent further operations if the user does not have the right permissions.
  if (!user_access('access content')) {
    drupal_access_denied();
  }
}