localization - PHP's gettext with en_US locale -

i have mo file

/data/lang/en_us/lc_messages/test.mo (translate foo => bar) 

and use following code echo it

$locale = 'en_us';  $locale_dir = '/data/lang'; putenv("lc_all=$locale"); setlocale(lc_all, $locale); bindtextdomain('test', $locale_dir); textdomain('test'); echo _("foo"); // print foo 

however, if change

$locale = 'en_us.utf8';  

then print corrected answer, bar

and system locale information:

locale -a .. en_us en_us.utf8 .. 

what reason?