I18n (short for internationalization) are means of adapting an application for different languages and cultures. The scope of i18n support includes support for local date and time, currency, measures, number formatting and strings in different encoding. Native support for i18n and l10n (localization) appeared in PHP6.
Please note that in PHP6, POSIX locales are obsoleted by the set of locales included with the ICU library.
This extension is still in development and it isn't available to public yet.
The Unicode Extension.