Example plugin for the presentation "WordPress hooks 
and plugin development" at the Larissa WordPress meetup

wplarissameetup.php

<?php
/*
 * Plugin Name: Plugin Development Larissa Meetup
 * Description: Intro to plugin development at WordPress Larissa Meetup
 * Plugin URI:  https://speakerdeck.com/wpgr/
 * Author:      Chris Ziogas
 * Author URI:  https://www.linkedin.com/in/ziogaschr/
 * Version:     1.0
 */

defined('ABSPATH') or die();

/*
 * Init action
 * http://codex.wordpress.org/Function_Reference/add_action
 */
function pdlm_init()
{
    if (is_admin() ) {
        return;
    }

    $pdlm_convert_links = get_option('pdlm_convert_links', 0);
    if ($pdlm_convert_links) {
        add_filter('the_content', 'pdlm_convert_text_links_to_html_links');
    }

    $pdlm_disable_emoji = get_option('pdlm_disable_emoji', 0);
    if ($pdlm_disable_emoji) {
        remove_action('wp_head', 'print_emoji_detection_script', 7);
        remove_action('wp_print_styles', 'print_emoji_styles');
    }
}
add_action('init', 'pdlm_init');

/*
 * Add add_menu page
 * http://codex.wordpress.org/Function_Reference/add_menu_page
 */
function pdlm_admin_menu()
{
    add_menu_page('Plugin Development - Larissa Meetup',
        'Larissa Meetup',
        'manage_options',
        'plugin_development_larissa_meetup',
        'plugin_development_larissa_meetup_settings');
}
add_action('admin_menu', 'pdlm_admin_menu');

/*
 * add_menu_page callback
 */
function plugin_development_larissa_meetup_settings()
{
    // save the submission
    if (isset($_GET['pdlm_save'])) {
        update_option('pdlm_convert_links', intval($_POST['pdlm_convert_links']));
        update_option('pdlm_disable_emoji', intval($_POST['pdlm_disable_emoji']));

        // redirect back to form
        wp_safe_redirect($_SERVER['HTTP_REFERER']);
        die();
    }

    // show the form
    plugin_development_larissa_meetup_settings_form();
}

/*
 * Settings page form
 */
function plugin_development_larissa_meetup_settings_form()
{
    $pdlm_convert_links = get_option('pdlm_convert_links', 0);
    $pdlm_disable_emoji = get_option('pdlm_disable_emoji', 0);
    ?>

  <div class="wrap">
    <h1>Plugin Development - Larissa Meetup</h1>
    <form action="<?php echo $_SERVER['PHP_SELF']; ?>?page=plugin_development_larissa_meetup&noheader=true&pdlm_save=true"
          method="post">
      <div>
        <label for="pdlm_convert_links">
            <input type="checkbox" id="pdlm_convert_links" name="pdlm_convert_links" value="1"
                <?php if ($pdlm_convert_links == 1) {
        echo ' checked="checked"';
    }
    ?>
            />
            Μετατροπή διευθύνσεων μέσα στο κείμενο σε HTML συνδέσμους.
        </label>

        <br />

        <label for="pdlm_disable_emoji">
            <input type="checkbox" id="pdlm_disable_emoji" name="pdlm_disable_emoji" value="1"
                <?php if ($pdlm_disable_emoji == 1) {
        echo ' checked="checked"';
    }
    ?>
            />
            Απενεργοποιήση WordPress εικόνων emoji, και χρήση χαρακτήρων unicode.
        </label>
      </div>

      <?php submit_button();?>
    </form>
  </div>
  <?php
}

/*
 * Convert text links into HTML links
 */
function pdlm_convert_text_links_to_html_links($content)
{
    return make_clickable($content);
}