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if (!empty($_GET['lang']) {
   // dein Code hier
}

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$browser = $_SERVER["HTTP_ACCEPT_LANGUAGE"];
$lang = substr($browser,0,2);

if($lang == "de") {
echo "<meta http-equiv='refresh' content='0; URL=http://www.xyz.de/index.php?lang=de'>";
}
elseif($lang == "pl") {
echo "<meta http-equiv='refresh' content='0; URL=http://www.xyz.det/index.php?lang=pl'>";
}
elseif($lang == "fr") {
echo "<meta http-equiv='refresh' content='0; URL=http://www.xyz.de/index.php?lang=fr'>";
}

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$browser = $_SERVER["HTTP_ACCEPT_LANGUAGE"];
$lang = substr($browser,0,2);

if($lang == "de") {
header("Refresh: 0; http://www.xyz.de/index.php?lang=de'");
}
elseif($lang == "pl") {
header("Refresh: 0; http://www.xyz.de/index.php?lang=po'");
}
elseif($lang == "fr") {
header("Refresh: 0; http://www.xyz.de/index.php?lang=fr'");
}