<?php
function GetPreferredLang($lang_accept, $lang_list) {
	// 1.0 beta - Not yet for general dissemination
	// http://www.anaerob.dk
	// GPL2/3 text here

	/**
	Returns the preferred language of the visitor

	http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
	http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.9

	Return values:
	----------
	0 => 'ok' - Success
	1 => $preferred_lang

	0 => 'nomatch' - No match, use default
	1 => $lang_list_org[0]

	0 => 'none' - No match, all unacceptable

	0 => 'error' - Failure (bad input etc.)
	1 => $reason
	----------

	$lang_accept: "da, en-gb;q=0.82, en;q=0.7, fr, de;q=0.009, zh;q=0"
	$lang_list: array("en", "fr")
	selects: "fr"

	Ranking Algorithm:
	Lang  : quality  padding  spec_bonus  position  0 =  Weight (Rank)
	da    :   1          000           0         4  0 = 1000040    (1)
	en-gb :   0.82         0           1         3  0 =  820130    (3)
	en    :   0.7         00           0         2  0 =  700020    (4)
	fr    :   1          000           0         1  0 = 1000010    (2)
	de    :   0.009                    0         0  0 =    9000    (5)
	zh    :   0            -           -         -  - =       -    (-)

	To Do:
	Clean-up
	Validation of input
	Testcases
	*/

	$error = FALSE;

	$lang_list_org = $lang_list; // Make a copy for future use

	$lang_accept = array_reverse(array_map('trim',explode(',', $lang_accept)));

	foreach ($lang_accept as $key => $value) {
		$value = explode(';q=', $value);
		// If a quality value hasn't been set explicitly it defaults to 1
		if (!isset($value[1]) && $value[0] != '*') {
			$value[1] = 1;
		}

		if ($value[0] == '*') {
			$catchall = TRUE;
			$value[1] = 0; // To avoid error messages
		}

		// Pad $value[1] (q)
		if ($value[1] == 1) {
			$value[1] = 1000;
		}
		else {
			$value[1] = str_replace('0.', '', $value[1]); // Make fractions (0.8) into integers (8)
			$value[1] = str_pad($value[1], 3, 0);
		}

		// 'en-gb' is more specific than 'en', so give more weight
		strpos($value[0], '-') !== FALSE ? $spec_bonus = 1 : $spec_bonus = 0;

		// Assemble the rank
		$rank = $value[1].$spec_bonus.$key. 0; // The 0 is to make sure that there's always a difference of 10 between each lang

		// Separate acceptable from unacceptable languages
		if ($value[1] != 0 && $value[0] != '*') { // * is a "last resort" selector that is only needed if no match is found, so no reason to have it in the list
			$lang_ranks[$value[0]] = (int)$rank;
		}
		elseif ($value[1] == 0 && $value[0] != '*') {
			$unacceptable[] = $value[0];
		}
	}
	unset($key);
	unset($value);
	// Avoids trouble
	if (!isset($unacceptable)) {
		$unacceptable = array();
	}

	// We do a sort now so that we get the correct (lowest) ranks in the fallback list
	arsort($lang_ranks);

	// Create a fallback list of language codes such as 'en' by reducing compound codes ('en-gb') to just the language part ('en')
	// Used when there is no general fallback in the main list ('en-gb, fr' -> 'en-gb, en, fr')
	foreach ($lang_ranks as $key => $value) {
		if (strpos($key, '-') !== FALSE) {
			 $fallback[substr($key, 0, 2)] = $value - 1; // We subtract 1 so that it will be lower in the list
		}
	}
	unset($key);
	unset($value);
	// Avoids trouble
	if (!isset($fallback)) {
		$fallback = array();
	}

	// Add the fallback list to the main one. array_merge() seems to be broken (php 5.0.4) (or I'm a dork), so we do this instead
	foreach ($fallback as $key => $value) {
		if (!array_key_exists($key, $lang_ranks)) {
			$lang_ranks[$key] = $value;
		}
	}
	unset($key);
	unset($value);

	// Remove any unacceptable languages from $lang_list
	$lang_list = array_diff($lang_list, $unacceptable);

	arsort($lang_ranks);
	$lang_ranks = array_flip($lang_ranks);

	// Keep only the languages that we can actually use
	$lang_ranks = array_intersect($lang_ranks, $lang_list);

	$preferred_lang = reset($lang_ranks);

	// An acceptable match has been found
	if (($preferred_lang != FALSE) && (count($lang_list) != 0) && ($error == FALSE)) {
		$status = 'ok';
	}
	// No match was found, so we simply return the first in $lang_list
	elseif (($preferred_lang == FALSE) && (count($lang_list) != 0) && ($error == FALSE)) {
		$preferred_lang = reset($lang_list);
		$status = 'nomatch';
	}
	// No match and $lang_list contains no acceptable languges
	elseif (($preferred_lang == FALSE) && (count($lang_list) == 0) && ($error == FALSE)) {
		$status = 'none';
	}

	if ($status == 'ok') {
		$foo = array(0 => 'ok', 1 => $preferred_lang);
	}
	if ($status == 'nomatch') {
		$foo = array(0 => 'nomatch', 1 => $preferred_lang);
	}
	if ($status == 'none') {
		$foo = array(0 => 'none');
	}
	if ($error != FALSE) {
		$foo = array(0 => 'error', 1 => $error_reason);
	}

	return $foo;
}
?>