PHP Visitor Language Detection per HTTP/1.1 Spec

This is Part 2 of Visitor Language Detection. Part 1 discussed the HTTP/1.1 specification. Here we have the PHP code for detecting the language to serve at your site as a combination of the browser language settings and your language offerings. In Part 3 is an example of your site's language settings as it's done in the phpGedView package.

/**
  * Copyright 2009 John Findlay, Tom Haws, and others
  * Licensed under GNU GPL v3.0 or later
  *
  * Get the language to use for this session.
  * See HTTP/1.1 Spec:
  * A language-range matches a language-tag if it exactly equals the tag, 
  * or if it exactly equals a prefix of the tag such that the first tag 
  * character following the prefix is "-". The special range "*", 
  * if present in the Accept-Language field, matches every tag not matched 
  * by any other range present in the Accept-Language field.
  * The language quality factor assigned to a language-tag by the 
  * Accept-Language field is the quality value of the longest language-
  * range in the field that matches the language-tag. If no language-
  * range in the field matches the tag, the language quality factor assigned 
  * is 0. If no Accept-Language header is present in the request, the server 
  * SHOULD assume that all languages are equally acceptable.
  * Get the string list of supported languages for this Browser's preferred page languages
  * "es,en-gb;q=0.9,en-us;q=0.8,en;q=0.6,pt;q=0.5,ie;q=0.4,it;q=0.3,fr;q=0.1"
  * 
  * Requires the global $all_language_settings variable as an argument.
  * 
  */
function getLanguage($all_language_settings) {
// if (empty($_SESSION["CLANGUAGE"])) { //(Comment this out. Let user change languages mid-stream)
    if (isset($HTTP_ACCEPT_LANGUAGE)) $accept_langs = $HTTP_ACCEPT_LANGUAGE;
    else if (!empty($_SERVER['HTTP_ACCEPT_LANGUAGE'])) $accept_langs = $_SERVER['HTTP_ACCEPT_LANGUAGE'];
    // If there is a browser language list
    if (isset($accept_langs)) {
      // Explode the list by commas
      $accept_langs_array = explode(",", $accept_langs);
      // If there is an Accept-Language header, the default language quality is 0.
      $browserDefaultQuality = 0;
      // Split each language entry into range and quality.
      foreach ($accept_langs_array as $key => $browserLang) {
          // Get the name, quality, and prefix (prefix is a hack for non-compliance) of the browser language preference.
          $browserLang = explode(";", strtolower($browserLang));
          $browserLang[1] = substr($browserLang[1],2);
          $browserLang[2] = explode("-", $browserLang[0]);
          $browserLang[2] = $browserLang[2][0];
          // If the range is "*", it gives the default language quality.
          if ($browserLang[0] == "*") $browserDefaultQuality = $browserLang[1];
          // If the range has no quality, the quality is 1.
          if ($browserLang[1] == "") $browserLang[1] = "1";
          $accept_langs_array[$key] = $browserLang;
      }
    } 
    else {
      // If no header, all languages are quality 1.
      $browserDefaultQuality = 1;
    }
    // First accepted language with a zero quality will be the default winner.
    $highestQuality=-1; 
    // Assign a combined quality * preference to each of our languages
    // print_r($all_language_settings);
    foreach ($all_language_settings as $tag => $language) {
      $tagarray = explode("-", $tag);
      $tagPrefix = $tagarray[0];
      $tagQuality = $language['QUALITY'];
      $longestMatch = 0;
      // Assign the default quality.
      $language['QUALITY'] = $browserDefaultQuality * $tagQuality;
      // If there is an Accept-Language header, adjust 
      //language quality according to it.
      if (isset($accept_langs)) {
        // Loop through the browser language preferences to get 
        // the browser's quality for tag
        foreach ($accept_langs_array as $browserLang) {
          // If the browser language range exactly equals 
          // our language tag or our prefix
          if ($browserLang[0] == $tagPrefix ||  $browserLang[0] == $tag) {
            // We get to use its quality if it's the longest range that matches.
            if (strlen($browserLang[0]) > $longestMatch) {
              $longestMatch = strlen($browserLang[0]);
              // Assign it in case this is the longest match.
              $language['QUALITY'] = $browserLang[1] * $tagQuality;
            }
          /**
           * NON-COMPLIANT hack for disinterested users that prefer 
           * to accept prefix language without saying so.
           * (They say es-ar, but really, really also prefer es to en)
           * (But not, on the negative side, if they say they hate 
           *  en-cockney, but really hate all forms of english.)  
           * In real world usage, the negative non-acceptance case is 
           * likely a sign of genuine user intent and attention.
           * An intentional user in such a situation would naturally 
           * assume generic en would NOT be included in his non-acceptance
           * of en-cockney. So we make sure we count such pseudo-matches
           * only for range qualities over 0.5. Also, we call it a 
           * pseudo-match with length of one since it's not
           * legitimate. We will be assigning its adjusted range quality 
           * to our tag only if there is no legitimate match.
           * 
           */
          } elseif (($browserLang[2] == $tagPrefix) && ($browserLang[1] > 0.5) && ($longestMatch < 1)) {
            // Call it a one character long match (a pseudo-match).
            $longestMatch = 1;
            // Assign it in case this is the longest match.
            $language['QUALITY'] = $browserLang[1]  * $tagQuality;
          }
          // echo "\n
For tag $tag, quality $tagQuality, browser range $browserLang[0], quality $browserLang[1], put tag quality at $language[QUALITY].";
        }
      }
      // If this tag has the highest quality so far, declare it the current winner.
      if ($language['QUALITY'] > $highestQuality) {
        $highestQuality = $language['QUALITY'];
        $winningLanguage=$tag;
      }
    }
    /*
    echo "Default browser quality: $browserDefaultQuality";
    print_r($accept_langs_array);
    print_r($language_settings);
    */
    $_SESSION["CLANGUAGE"]=$winningLanguage;
    return $winningLanguage;
// } else {
//  return $_SESSION["CLANGUAGE"];
// }
}

$clanguage=getLanguage($all_language_settings);
?>