Yesterday i wrote that you can use __($string, $domain) and _e($string, $domain) to internationalize a string with WordPress. I forgot to mention that if you use sprintf you can handle strings with variables too. An example:

echo sprintf(__('There are %d monkeys in the %s'), $domain), $number, $location);

WordPress uses GNU gettext, as explained in Translating WordPress and Writing a Plugin, for localization. I decided that my plugins should support l10n too. Here is how i realised it:

I started with defining the WPLANG constant in my wp-config.php:

define ('WPLANG', 'en_EN');

Then i changed the beginning of my localized plugin as following:

<?php
/*
Plugin Name: WP-SpamFilter
Version: 0.4
Plugin URI: http://timvw.madoka.be/?p=533
Description: Mark the new comment as spam if the sender is in the spammers list.
Author: Tim Van Wassenhove <timvw@users.sourceforge.net>
Author URI: http://timvw.be
*/

// If this function does not exist it means that the file is accessed directly.
// Accessing this file directly is not allowed.
if (!function_exists('load_plugin_textdomain')) {
  exit;
}

load_plugin_textdomain('wp_spamfilter', 'wp-content/plugins/wp-spamfilter');

In wp-includes/wp-l10n.php you see that the result of this call is that the locale and the path are used to determine the mo-file to be loaded. Because this function uses ABSPATH it’s impossible to use dirname(__FILE__) as path parameter

The _e($string, $domain) function echos a localized string and the __($string, $domain) function returns a localized string that you can use in function calls etc… Below you can see an example of the original version and the updated version with localization support:

<h2>some string</h2>
<?php echo('some other string'); ?>

<h2><?php _e('some string', 'wp_spamfilter'); ?></h2>
<?php echo(__('some other string', 'wp_spamfilter')); ?>

Then i used xgettext to extract all the strings that should be localized into wp_spamfilter-en_EN.po:

xgettext –keyword=__ –keyword=_e –default-domain=wordpress –language=php *.php –output=wp_spamfilter-en_EN.po

After that i editted the po file and i compiled a mo file with it using msgfmt:

msgfmt wp_spamfilter-en_EN.po -o wp_spamfilter-en_EN.mo

Once i had made sure that wp_spamfilter-en_EN.mo was readable by my webserver i was ready.

Today i’ve been cleaning up my code. All my plugins live in a directory %plugin_name% under the wp-content/plugins directory. Here is the code for %plugin_name%/%plugin_name%.php:

<?php
/*
Plugin Name: %plugin_name%
Version: %plugin_version%
Plugin URI: %plugin_uri%
Description: %plugin_description%
Author: %plugin_author_name% <%plugin_author_mail%>
Author URI: %plugin_author_uri%
*/

// Direct access is not allowed.
if (!function_exists('load_plugin_textdomain')) {
  exit;
} else {
  require('%plugin_name%.php');
  $plugin_name = substr(basename(__FILE__), 0, -4);

  // You can add optional parameters to the constructor, eg: the wpdb instance.
  $%plugin_class% = new %plugin_class%($plugin_name);
}
?>

Now it’s time to implement the %plugin_class%. Here is the template for %plugin_name%/%plugin_class%.php:

<?php

class %plugin_class% {
  var $_plugin_name;

  var $_localization_domain;
  var $_localization_path;

  function %plugin_class%($plugin_name = '%plugin_name%') {
    $this->_plugin_name = $plugin_name;

    $this->_localization_domain = $plugin_name;
    $this->_localization_path = 'wp-content/plugins/' . $plugin_name;

    // You can add additional hooks and filters here.
    add_action('activate_' . $plugin_name . '/' . $plugin_name . '.php', array(&$this, 'OnActivation'));
    add_action('admin_menu', array(&$this, 'OnAdminMenu'));
}

  function OnAdminMenu() {
    load_plugin_textdomain($this->_localization_domain, $this->_localization_path);
    add_options_page(__('%plugin_name% Options', $this->_localization_domain), __('%plugin_name%', $this->_localization_domain), 'manage_options',$this->_plugin_name . '/' . $this->_plugin_name . '-options.php', array(&$this, 'OnDisplayOptions'));
  }

  function OnDisplayOptions() {
    $%plugin_class% = "";
    require( dirname(__FILE__) . '/' . $this->_plugin_name . '-options.php');
  }

  function OnActivation() {
    // This code is executed when the plugin is activated.
    // I prepend all my option names with $this->_plugin_name.
    add_option($this->_plugin_name . '_somevar', 'foo');
  }
}
?>

The code above requires that you create a %plugin_name%/%plugin_name%-options.php file to administrate the options. Here is the template for that file:

<?php
// Direct access is not allowed.
if (!isset($%plugin_class%)) {
  exit;
}

load_plugin_textdomain($%plugin_class%->_localization_domain, $%plugin_class%->_localization_path);

// Handle post action.
if ($_POST['stage'] == 'process') {
  // All the names of form variables start with %plugin_class%_plugin_name.
  if (isset($_POST[$%plugin_class%->_plugin_name . '_somevar'])) {
    // Do processing here.
  }
}

?>
<form method="post" action="">
  <input type="hidden" name="stage" value="process"/>
  <input type="text" name="<?php echo $%plugin_class%->_plugin_name . '_somevar'; ?>"/>
  <input type="submit" value="<?php _e('Submit', $%plugin_class%->_localization_domain); ?>"/>
</form>

The only thing that we still have to do is generate a po file with xgettext, translate it and compile a %plugin_name%/%plugin_name%-en_EN.mo file.

The WordPress documentation says that you have to call add_action(‘activate_pluginurl’, ‘somefunction’) to trigger somefunction when your plugin is activated. Unfortunately i couldn’t find with what pluginurl should be replaced. After a bit of experimenting i’ve found that in wp-admin/plugins.php the following is called when a plugin is activated:

do_action('activate_' . trim( $_GET['plugin'] ));

So it appears that you simply have to use the path of your plugin relative to /wp-content/plugins. Eg: you have a plugin in /wp-content/plugins/wp-spamfilter/wp-spamfilter.php then you have to call add_action as following:

add_action('activate_wp-spamfilter/wp-spamfilter.php', 'somefunction');