This is a very minimal plugin that does nothing but increase your site's Geek Factor.

Directory Structure

mantis/
  plugins/
    supercow/
      register.php
      events.php

sample/register.php

<?php

/**
 * Return plugin details to the API.
 * @return array Plugin details
 */
function plugin_callback_supercow_info() {
  return array(
    'name' => 'Super Cow Powers',
    'description' => 'Gives your Mantis installation super cow powers.',
    'version' => '1.0',
    'author' => 'John Reese',
    'contact' => 'jreese@leetcode.net',
    'url' => 'http://leetcode.net',
    'page' => '',
    'requires' => array(),
  );
}

/**
 * Intitialize the plugin.
 */
function plugin_callback_supercow_init() {
  plugin_event_hook( 'EVENT_PLUGIN_INIT', 'header' );
}

sample/events.php

<?php

/**
 * Handle the EVENT_PLUGIN_INIT callback.
 */
function plugin_event_supercow_header() {
  header( 'X-Mantis: This Mantis has super cow powers.' );
}