set_locales en_US fails

i'm trying 开发者_Go百科to set locales to en_US, but it returns nothing?!

setLocale(LC_ALL, 'en_US');

is it the right code i'm using?

The en_US locale may not be installed on your system, or your system doesn't support locales at all. See documentation:

Returns the new current locale, or FALSE if the locale functionality is not implemented on your platform, the specified locale does not exist or the category name is invalid.