gettext in php always returns input string - php

Question

I need to get my php code multilingual. The translations are POEdit.com exports.
Because I want to display single and plurals I have to use gettext (the php array export of POEditor does not support plurals).
My problem:
Gettext does always return the input string (msgid), not the translated text.
My setup:
Windows 7 64-Bit german system locale
EasyPHP Devserver
Apache 2.4.18
PHP 5.6.17 + gettext enabled
Folders and Files:
web root = C:\Users\SM\PhpstormProjects\website
locales = C:\Users\SM\PhpstormProjects\website\src\i18n
polish translation = C:\Users\SM\PhpstormProjects\website\src\i18n\plk\LC_MESSAGES\messages.mo
PHP Code:
define('_LOCALES_DIR', 'C:\Users\SM\PhpstormProjects\website\src\i18n');
$locale = "plk";
$domain = "messages";
$rv['putenv'] = putenv("LC_ALL=" . $locale);
$rv['setlocale'] = setlocale(LC_ALL, $locale);
$rv['bindtextdomain'] = bindtextdomain($domain, _LOCALES_DIR);
$rv['bind_textdomain_codeset'] = bind_textdomain_codeset($domain,"UTF-8");
$rv['textdomain'] = textdomain($domain);
print_r($rv);
echo "pl: " . gettext("Benutzername");
PHP Output:
Array (
[putenv] => 1
[setlocale] => Polish_Poland.1250
[bindtextdomain] => C:\Users\SM\PhpstormProjects\website\src\i18n
[bind_textdomain_codeset] => UTF-8
[textdomain] => messages
)
pl: Benutzername
Expected output: "pl: użytkownik"
The .po file looks like this:
msgid ""
msgstr ""
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"X-Generator: POEditor.com\n"
"Project-Id-Version: website\n"
"Language: pl\n"
"Plural-Forms: nplurals=3; plural=(n==1 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2);\n"
#:
msgctxt "Usermanagement"
msgid "E-Mail"
msgstr "e-mail"
#:
msgctxt "Usermanagement"
msgid "Passwort"
msgstr "hasło"
#:
msgctxt "Usermanagement"
msgid "Benutzername"
msgstr "użytkownik"
What I tried:
The return value of the setlocale("plk") ist "Polish_Poland.1250" so I tried to rename the "i18n/plk" folder to "i18n/Polish_Poland.1250" without luck.
use Polish_Poland.1250 as input language
Polish is only an example, tried it with different languages
added ".UTF8" and ".UTF-8" to the locale
tried php-gettext
checked the installed locales with the help of this script: https://stackoverflow.com/a/23286243
But always the same result :(
Does one of you have any ideas what am I doing wrong or could trie to get it to work?
UPDATE 2016-07-15 12:45:
One of my mistakes is the missing context, I had to use gettext() like this gettext("Usermanagement\004Benutzername");. Now it loads text from the mo files, but the wrong one. It always loads the mo file located in the "i18n/de_DE" folder. (Remember de_DE is the default language of my Windows installation). Even setlocale returns the entered locale (setlocales way to tell he successfully changed the locale), always de_DE is used. :/
I also tried additionally to set these to environment variables without any luck:
$rv['putenv=LANG'] = putenv('LANG='.$locale);
$rv['putenv=LANGUAGE'] = putenv('LANGUAGE='.$locale);