textdomain-overwrite.php

<?php
/*
 * Plugin Name: Textdomain Overwrite
 * Description: Load custom localization files to overwrite core, plugin and theme strings
 * Version: 1.0.1
 * Author: Bernhard Kau
 * Author URI: http://kau-boys.de
 * License: GPLv3
 * License URI: http://www.gnu.org/licenses/gpl-3.0
 */

class Textdomain_Overwrite {

	// used to store the current locale e.g. "de_DE"
	private $locale;

	// used to store the folder with the overwrite files
	private $overwrite_folder;

	// flag to check if running a multisite environment
	private $is_multisite;

	// used to store the multisite blog_id
	private $current_blog_id;

	function __construct() {

		// get current locale
		$this->locale = get_locale();

		// set folder for overwrites
		$this->overwrite_folder = WP_LANG_DIR . '/overwrites/';

		// check if multisite
		$this->is_multisite = is_multisite();

		// if it is a multisite, get the current blog_id
		if ( $this->is_multisite ) {
			$this->current_blog_id = get_current_blog_id();
		}

		// register action that is triggered, whenever a textdomain is loaded
		add_action( 'override_load_textdomain', array( $this, 'overwrite_textdomain' ), 10, 3 );
	}

	/*
	 * Overwriting strings from all loaded textdomains, no matter if they are used in Core, Plugins or Themes
	 *
	 * The .mo file has to be named [domain]-[locale].mo
	 * e.g. for the plugin Jetpack with the textdomain "jetpack"
	 * and the locale "de_DE" is has to be jetpack-de_DE.mo
	 */
	function overwrite_textdomain( $override, $domain, $mofile ) {

		// if the filter was not called with an overwrite mofile, return false which will proceed with the mofile given and prevents an endless recursion
		if ( strpos( $mofile, $this->overwrite_folder ) !== false ) {
			return false;
		}

		// if an overwrite file exists, load it to overwrite the original strings
		$overwrite_mofile = $domain . '-' . $this->locale . '.mo';

		// check if a global overwrite mofile exists and load it
		$global_overwrite_file = $this->overwrite_folder . $overwrite_mofile;

		if ( file_exists( $global_overwrite_file ) ) {
			load_textdomain( $domain, $global_overwrite_file );
		}

		// check if a overwrite mofile for the current multisite blog exists and load it
		if ( $this->is_multisite ) {
			$current_blog_overwrite_file = $this->overwrite_folder . 'blogs.dir/' . $this->current_blog_id . '/' . $overwrite_mofile;

			if ( file_exists( $current_blog_overwrite_file ) ) {
				load_textdomain( $domain, $current_blog_overwrite_file );
			}
		}

		return false;
	}

}

new Textdomain_Overwrite;