<?php

$locale = "fa";

// Define location and domain of translations
$directory = realpath('./') .'/translations/LOCALES';
$domain    = 'php-addressbook';
$default_locale = $locale;
$supported_locales = array($locale);


// Prepare "php-gettext"
require_once('gettext/gettext.inc');


// Prepare "native gettext" setup
T_setlocale(LC_ALL, $locale);
T_bindtextdomain($domain, $directory);
T_textdomain($domain);
T_bind_textdomain_codeset($domain, 'UTF-8');

echo T_gettext('ADDRESS');

if (!locale_emulation()) {
    print "<p>locale '$locale' is supported by your system, using native gettext implementation.</p>\n";
}
else {
    print "<p>locale '$locale' is _not_ supported on your system, using the default locale '". $default_locale ."'.</p>\n";
}

?>

<?php
  
error_reporting(E_ALL | E_STRICT);

$locale = "fa";

//
// Define location and domain of translations
//
$directory = realpath('./') .'/translations/LOCALES';
$domain    = 'php-addressbook';
$default_locale = "en";

//
// Prepare "native gettext"
//
putenv("LANG=".$locale);
setlocale(LC_ALL, $locale);
bindtextdomain($domain, $directory);
textdomain($domain);
if(function_exists('bind_textdomain_codeset')) {
  bind_textdomain_codeset($domain, 'UTF-8');
}
textdomain($domain);


//
// Prepare "php-gettext"
//
require_once('gettext/gettext.inc');

define('PROJECT_DIR', realpath('./'));
define('LOCALE_DIR',  $directory);
define('DEFAULT_LOCALE', $default_locale);

echo gettext('ADDRESS');
?>

Works, even if a locale is not avaliable on the system.

PHP gettext with fallback for native not supported languages

php

PHP