PHP utility class to get the preferred language of the current user. The language will be read from the HTTP AcceptLanguage, but can be overridden by GET and POST parameters. Preferences can also be saved in a cookie.

LanguageUtils.php

<?php

class LanguageUtils {

    public static $COOKIE_LIFETIME = 10 * 365 * 24 * 3600; // 10 years
    public static $LANGUAGE_KEY = "lang"; // key used as cookie name and to check $_POST and $_GET

    /**
     * Get the user language based on the following priorities:
     * - "$LANGUAGE_KEY" parameter in $_POST data
     * - "$LANGUAGE_KEY" parameter in $_GET data
     * - "$LANGUAGE_KEY" cookie
     * - preferred language from AcceptLanguage http header
     * - default language
     * @param array $supportedLanguages the language codes supported by the application
     * @param string $default the fallback language code
     * @param string|null $acceptLanguage accept language header. Defaults to  $_SERVER['HTTP_ACCEPT_LANGUAGE']
     * @param bool $saveGetPostAsCookie if set to true, if the LANGUAGE_KEY is found, in $_GET or $_POST data, it will be saved in a cookie
     */
    public static function getUserLanguage(array $supportedLanguages, $default = "en", $acceptLanguage = null, bool $saveGetPostAsCookie = false) {

        if (array_key_exists(self::$LANGUAGE_KEY, $_POST) && in_array($_POST[self::$LANGUAGE_KEY], $supportedLanguages)) {
            $lang = $_POST[self::$LANGUAGE_KEY];
            if ($saveGetPostAsCookie) self::saveUserPreference($lang);
            return $lang;
        }

        if (array_key_exists(self::$LANGUAGE_KEY, $_GET) && in_array($_GET[self::$LANGUAGE_KEY], $supportedLanguages)) {
            $lang = $_GET[self::$LANGUAGE_KEY];
            if ($saveGetPostAsCookie) self::saveUserPreference($lang);
            return $lang;
        }

        if (array_key_exists(self::$LANGUAGE_KEY, $_COOKIE) && in_array($_COOKIE[self::$LANGUAGE_KEY], $supportedLanguages)) {
            return $_COOKIE[self::$LANGUAGE_KEY];
        }

        if (!$acceptLanguage) $acceptLanguage = $_SERVER['HTTP_ACCEPT_LANGUAGE'];
        $preferred_language = self::preferred_language($supportedLanguages, $acceptLanguage);
        return $preferred_language !== null ? $preferred_language : $default;
    }

    public static function saveUserPreference($lang) {
        setcookie(self::$LANGUAGE_KEY, $lang, time() + self::$COOKIE_LIFETIME);
    }

    /**
     * source: https://gist.github.com/Xeoncross/dc2ebf017676ae946082
     */
    private static function preferred_language(array $available_languages, string $http_accept_language) {
        $available_languages = array_flip($available_languages);

        $langs = array();
        preg_match_all('~([\w-]+)(?:[^,\d]+([\d.]+))?~', strtolower($http_accept_language), $matches, PREG_SET_ORDER);
        foreach($matches as $match) {

            list($a, $b) = explode('-', $match[1]) + array('', '');
            $value = isset($match[2]) ? (float) $match[2] : 1.0;

            if(isset($available_languages[$match[1]])) {
                $langs[$match[1]] = $value;
                continue;
            }

            if(isset($available_languages[$a])) {
                $langs[$a] = $value - 0.1;
            }

        }

        if($langs) {
            arsort($langs);
            return key($langs); // We don't need the whole array of choices since we have a match
        }
    }
}

test.php

<?php
// Example Usage:
$language = LanguageUtils::getUserLanguage(["de", "en"], "en", null, true);
?>