load_textdomain()

load_textdomain( string $domain, string $mofile, string $locale = null ): bool

function load_textdomain( $domain, $mofile, $locale = null ) {
	/** @var WP_Textdomain_Registry $wp_textdomain_registry */
	global $l10n, $l10n_unloaded, $wp_textdomain_registry;

	$l10n_unloaded = (array) $l10n_unloaded;

	if ( ! is_string( $domain ) ) {
		return false;
	}

	/**
	 * Filters whether to short-circuit loading .mo file.
	 *
	 * Returning a non-null value from the filter will effectively short-circuit
	 * the loading, returning the passed value instead.
	 *
	 * @since 6.3.0
	 *
	 * @param bool|null   $loaded The result of loading a .mo file. Default null.
	 * @param string      $domain Text domain. Unique identifier for retrieving translated strings.
	 * @param string      $mofile Path to the MO file.
	 * @param string|null $locale Locale.
	 */
	$loaded = apply_filters( 'pre_load_textdomain', null, $domain, $mofile, $locale );
	if ( null !== $loaded ) {
		if ( true === $loaded ) {
			unset( $l10n_unloaded[ $domain ] );
		}

		return $loaded;
	}

	/**
	 * Filters whether to override the .mo file loading.
	 *
	 * @since 2.9.0
	 * @since 6.2.0 Added the `$locale` parameter.
	 *
	 * @param bool        $override Whether to override the .mo file loading. Default false.
	 * @param string      $domain   Text domain. Unique identifier for retrieving translated strings.
	 * @param string      $mofile   Path to the MO file.
	 * @param string|null $locale   Locale.
	 */
	$plugin_override = apply_filters( 'override_load_textdomain', false, $domain, $mofile, $locale );

	if ( true === (bool) $plugin_override ) {
		unset( $l10n_unloaded[ $domain ] );

		return true;
	}

	/**
	 * Fires before the MO translation file is loaded.
	 *
	 * @since 2.9.0
	 *
	 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
	 * @param string $mofile Path to the .mo file.
	 */
	do_action( 'load_textdomain', $domain, $mofile );

	/**
	 * Filters MO file path for loading translations for a specific text domain.
	 *
	 * @since 2.9.0
	 *
	 * @param string $mofile Path to the MO file.
	 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
	 */
	$mofile = apply_filters( 'load_textdomain_mofile', $mofile, $domain );

	if ( ! $locale ) {
		$locale = determine_locale();
	}

	$i18n_controller = WP_Translation_Controller::get_instance();

	// Ensures the correct locale is set as the current one, in case it was filtered.
	$i18n_controller->set_locale( $locale );

	/**
	 * Filters the preferred file format for translation files.
	 *
	 * Can be used to disable the use of PHP files for translations.
	 *
	 * @since 6.5.0
	 *
	 * @param string $preferred_format Preferred file format. Possible values: 'php', 'mo'. Default: 'php'.
	 * @param string $domain           The text domain.
	 */
	$preferred_format = apply_filters( 'translation_file_format', 'php', $domain );
	if ( ! in_array( $preferred_format, array( 'php', 'mo' ), true ) ) {
		$preferred_format = 'php';
	}

	$translation_files = array();

	if ( 'mo' !== $preferred_format ) {
		$translation_files[] = substr_replace( $mofile, ".l10n.$preferred_format", - strlen( '.mo' ) );
	}

	$translation_files[] = $mofile;

	foreach ( $translation_files as $file ) {
		/**
		 * Filters the file path for loading translations for the given text domain.
		 *
		 * Similar to the 'load_textdomain_mofile' filter with the difference that
		 * the file path could be for an MO or PHP file.
		 *
		 * @since 6.5.0
		 * @since 6.6.0 Added the `$locale` parameter.
		 *
		 * @param string $file   Path to the translation file to load.
		 * @param string $domain The text domain.
		 * @param string $locale The locale.
		 */
		$file = (string) apply_filters( 'load_translation_file', $file, $domain, $locale );

		$success = $i18n_controller->load_file( $file, $domain, $locale );

		if ( $success ) {
			if ( isset( $l10n[ $domain ] ) && $l10n[ $domain ] instanceof MO ) {
				$i18n_controller->load_file( $l10n[ $domain ]->get_filename(), $domain, $locale );
			}

			// Unset NOOP_Translations reference in get_translations_for_domain().
			unset( $l10n[ $domain ] );

			$l10n[ $domain ] = new WP_Translations( $i18n_controller, $domain );

			$wp_textdomain_registry->set( $domain, $locale, dirname( $file ) );

			return true;
		}
	}

	return false;
}