If your PO/MO files make use of msgctxt (Context), you'll be frustrated to find that gettext(msgid) won't work as you might expect (even if msgid is unique).

In this case, you must use ASCII char 4 [EOT, End Of Text] glued between the msgctxt and msgid.

Example PO content:

    msgctxt "Context"
    msgid "Test string"
    msgstr "Test translation"

    msgid "Standard string"
    msgstr "Standard translation"

The following will work:

<?php
gettext('Context' . "\004" . 'Test string');
gettext('Standard string');
?>

The following will NOT work:

<?php
gettext('Test string');
?>

The simplest way to by-pass gettext() cache, without restart apache nor change domain.

The fix is incredible simple, first create a dummy link to the locale folder where .mo files stored:

cd locale
ln -s . nocache

Then add one single line before bindtextdomain()

<?php
bindtextdomain('domain', './locale/nocache');
bindtextdomain('domain', './locale');
?>

Now the cache is forced to flush every time.

Worth noting is that gettext honors environment variables while selecting the language to use: http://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html

"When a program looks up locale dependent values, it does this according to the following environment variables, in priority order:

    LANGUAGE
    LC_ALL
    LC_xxx, according to selected locale category: LC_CTYPE, LC_NUMERIC, LC_TIME, LC_COLLATE, LC_MONETARY, LC_MESSAGES, ...
    LANG 

Variables whose value is set but is empty are ignored in this lookup. "

In short, if you have non-empty LANGUAGE, you may end up with unexpected localization strings. On the other hand, LANGUAGE can be used to define fallback language if some translation is missing.

As of php 5.3, you can use the following code to get the preferred locale of the http agent.

<?php
$locale = Locale::acceptFromHttp($_SERVER['HTTP_ACCEPT_LANGUAGE']);
?>

Gettext translations are cached. If you change *.mo files your page may not be translated as expected. Here's simple workaround which does not require restarting webserver (I know, this is just a dirty hack):

<?php
function initialize_i18n($locale) {
    putenv('LANG='.$locale);
    setlocale(LC_ALL,"");
    setlocale(LC_MESSAGES,$locale);
    setlocale(LC_CTYPE,$locale);
    $domains = glob($locales_root.'/'.$locale.'/LC_MESSAGES/messages-*.mo');
    $current = basename($domains[0],'.mo');
    $timestamp = preg_replace('{messages-}i','',$current);
    bindtextdomain($current,$locales_root);
    textdomain($current);
    }
?>

to make this work you have to put your locale inside file messages-[unix_time].mo and use this name (without .mo) as your domain to fool caching mechanism (domain names differ)

msgfmt messages.po -o messages-`date +%s`.mo

for me this works fine (although this is not very elegant solution)

on OSX (10.9.3) and PHP (5.4.24) you need to use full local name including codeset

i.e. for German need to use de_DE.UTF-8 even setlocale returns success when used without .UTF-8 the lookups will not work.

Depending on the implementation of gettext used you might have to call the setlocale(LC_ALL, "") command.  
So your example code would be 

<?php

// Set language to German
putenv ("LANG=de");

// set the locale into the instance of gettext
setlocale(LC_ALL, "");

// Specify location of translation tables
bindtextdomain ("myPHPApp", "./locale");

// Choose domain
textdomain ("myPHPApp");

// Print a test message
print (gettext ("Welcome to My PHP Application"));
?>

NOTE:  If setlocale returns NULL the LANG specified is invalid and "not supported".

I just wanted to say that gettext won't work on WAMP Server 2.4 / 64 Bit, see the thread I posted here:

Title: Gettext doesn't work on WAMP 64 Bits 
http://forum.wampserver.com/read.php?2,120770,120770#msg-120770

I haven't tested with only apache 64 Bit, so, I don't know if the issue is related to apache or WAMP. Anyway, to make it work with WAMP, install the 32 Bit version of WAMP and only do this:

define('LOCALE_DIR', '<root_dir_of_your_po_files>'); //ie: C:/wamp/www/your_app/locale
$locale = '<your_locale>'; //ie: es_CO
$domain = 'your_gettext_domain'; //ie: messages
putenv('LC_ALL=' . $locale);
bindtextdomain($domain, LOCALE_DIR);
textdomain($domain);
echo _('<your_string>'; // ie: Hello world

"setlocale" nor setting the LANG, LANGUAGE, and LC_MESSAGES environment variables seems to be necessary under windows. I got it working by setting "LC_ALL" only.

If you come across a situation where the translation works fine on CLI but not on Apache, it may be caused by the Perl Apache module.

Basic translate.php:

<?php
// Set locale
putenv("LC_ALL=de_CH.UTF-8");
putenv("LANGUAGE=");
putenv("LANG=de_CH.UTF-8");
$res = setlocale(LC_ALL, 'de_CH.UTF-8', 'de_CH', 'de');
echo bindtextdomain("homepage", "./locale");
textdomain("homepage");

$results = gettext("My English Text");
if ($results === "My English Text") {
    echo "Original English was returned. Something wrong\n";
} else {
    echo $results;
}
?>

On the CLI the translation works:

$ php7.3 translate.php
Mein deutscher Text.

But not via Apache web server:

$ curl localhost/translate.php
Original English was returned. Something wrong

Disable Perl module and restart Apache:

# a2dismod perl
# systemctl restart apache2

And suddenly the translation works:

$ curl localhost/translate.php
Mein deutscher Text.

The exact reason for this behaviour is (as of right now) unknown to me.

There's a good tutorial to the GetText tools used with PHP at http://zez.org/article/articleview/42
The only modification I needed to do was to use the correct ISO-language/country-codes (don't know the ISO number) and call setlocale. 
helloworld.php:

<?php
putenv("LC_ALL=da_DK"); // For danish/Denmark
setlocale(LC_ALL, "");

// ./locale/da/LC_MESSAGES holds the helloworld.mo file
bindtextdomain("helloworld", "./locale");
textdomain("helloworld");

print(gettext("Hello world!"));
?>

I had a lot of trouble getting this to work on Red Hat (Yellow Dog) Linux though.

I was testing in a self-made NAS (Debian + OpenMediaVault) and I was not getting SOME translations.

Some translations worked OK but some others were ignored.

The problem was caused because I had not added that locales to my NAS locale:

1) Edit file /etc/locale.gen
2) Check if your locale is active (written and uncommented).
3) Execute "locale-gen" as root (or sudo).

So, in short, if you have a Linux system and your translations are ignore check if you have that locale active in your system.

putenv(...) can cause hidden problems when upgrading or moving between systems which are difficult to diagnose.

On one Linux server we had the following working perfectly with setlocale

putenv("LANG=$locale");

We switched servers and found gettext wouldn't work despite having all the same locale files and settings

The recommendation to switch to

putenv("LC_ALL=$locale");

didn't fix the problem.

However,

putenv("LANGUAGE=$locale");

did. So if you have problems, check all three settings.

For me it is sufficient to call setlocale() with a string like "nl_BE" as the second parameter, to make gettext() work. Just plain "nl" was not enough.

Ditto when using an environment variable like LANG: "en", "fr", "nl", "de" are not enough: I have to specify the country, too.

gettext returns the headers from .mo files
if the message parameter is set to empty.

So if you are for example using Smarty blocks, make sure that the values given checks if the text has content or else your text will have a bunch of headers printing.

If you are putting a variable to the gettext, like so:
_($text);
you are better of making another function like this:
<?php
function __($text){
if(empty($text)) return "";
else gettext($text);
}
?>

Take care when extracting the strings from the source files : if your source files are not encoded in ascii, then xgettext must be used with the --from-code option, and the generated .po file is *always* UTF-8 (even if you used a different --from-code charset). 

The usage of gettext will not work later on strings which include non ascii caracters. For make it working, you have to translate the .po file to your proper charset with msgconv.

Example :
my source files are encoded in iso-8859-1
$ xgettext --from-code=iso-8859-1 -n *.php -o myapp.po
==> myapp.po is in UTF-8 (and generated .mo files will not work with gettext). 
I have to convert it to iso-8859-1 before translating :
$ msgconv --to-code=iso-8859-1 myapp.po -o myapp.po
...and now translate the file.

If you 're experiencing problems like gettext() is not working and you're getting translated text only occassionaly use: unset LANG before starting apache.
Next thing is that you have to restart apache after you 've changed .mo files because they're treated something like shared libraries.
I've only tested this with Linux (Sourcemage Linux distro, Mandrake) but it might be true for others as well.