Parse the HTTP except language header

http_accept_language.php

<?php

function prefered_language($available_languages, $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
	}
}

index.php

<?php

//$_SERVER["HTTP_ACCEPT_LANGUAGE"] = 'en-us,en;q=0.8,es-cl;q=0.5,zh-cn;q=0.3';

// Languages we support
$available_languages = array("en", "zh-cn", "es");

$lang = prefered_language($available_languages, $_SERVER["HTTP_ACCEPT_LANGUAGE"]);

print "<pre>Debug\n";
print_r($available_languages);
print_r(explode(',', $_SERVER["HTTP_ACCEPT_LANGUAGE"]));
print $lang;

output.txt

// $available_languages
Array
(
    [0] => en
    [1] => zh-cn
    [2] => es
)
// $_SERVER["HTTP_ACCEPT_LANGUAGE"]
Array
(
    [0] => en-us
    [1] => en;q=0.8
    [2] => es-cl;q=0.5
    [3] => zh-cn;q=0.3
)
// Result
en