AcceptLanguage negotiation algorithm

accept-language.php

<?php
	/**
	 * Return accept languages
	 * @return array
	 *
	 * HTTP content negotiation (section 12) uses short "floating point" numbers to indicate the
	 * relative importance ("weight") of various negotiable parameters. A weight is normalized to
	 * a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value.
	 * If a parameter has a quality value of 0, then content with this parameter is `not acceptable'
	 * for the client. HTTP/1.1 applications MUST NOT generate more than three digits after the
	 * decimal point. User configuration of these values SHOULD also be limited in this fashion.
	 *
	 * qvalue         = ( "0" [ "." 0*3DIGIT ] )
	 *                | ( "1" [ "." 0*3("0") ] )
	 *
	 * "Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.
	 *
	 * @see http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.9
	 */
	public function getAcceptLanguages()
	{
		if ( isset($_SERVER['HTTP_ACCEPT_LANGUAGE']) === false )
		{
			return [];
		}

		if ( preg_match_all('/(?P<langcode>[A-Za-z-]+)(?:;q=(?P<quality>[0-9\.]+))?/', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $matches) == 0 )
		{
			return [];
		}

		$qualities = $matches['quality'];
		$languages = $matches['langcode'];
		$languages = array_map('strtolower', $languages);

		foreach ( $qualities as $k => $quality )
		{
			if ( $quality === '' or $quality > 1 )
			{
				$qualities[$k] = 1;
			}
			else
			{
				$qualities[$k] = ceil($quality * 1000) / 1000;
			}
		}

		$acceptLanguages = array_combine($languages, $qualities);
		$acceptLanguages = array_filter($acceptLanguages); // remove quality=0

		return $acceptLanguages;
	}

	/**
	 * Negotiate language
	 * @param string[] $serverLanguages Language codes must be lower case
	 * @param float[] $clientLanguages A Hush which keys are language codes and values are qualities
	 * @return string
	 */
	public function negotiateLanguage(array $serverLanguages, array $clientLanguages)
	{
		// Set up client language list.
		$orderPriority = 1;

		foreach ( $clientLanguages as $language => $clientPriority )
		{
			$clientLanguages[$language] = ( ( 1 - $clientPriority ) * 10000 ) + $orderPriority;
			$orderPriority += 1;

			if ( strpos($language, '-') > 0 )
			{
				$primaryLanguage = substr($language, 0, strpos($language, '-'));

				if ( isset($clientLanguages[$primaryLanguage]) === false )
				{
					// The priority will be increased by 1000 for partial matches.
					$clientLanguages[$primaryLanguage] = $clientLanguages[$language] + 1000;
				}
			}
		}

		// Here, the nearer to zero the priority is, the higher the quality is.
		asort($clientLanguages);

		// Set up server language list.
		$serverLanguages = array_combine($serverLanguages, $serverLanguages);

		foreach ( $serverLanguages as $serverLanguage )
		{
			if ( strpos($serverLanguage, '-') > 0 )
			{
				$primaryLanguage = substr($serverLanguage, 0, strpos($serverLanguage, '-'));

				if ( isset($serverLanguages[$primaryLanguage]) === false )
				{
					$serverLanguages[$primaryLanguage] = $serverLanguage;
				}
			}
		}

		// Negotiate.
		foreach ( $clientLanguages as $language => $priority )
		{
			if ( isset($serverLanguages[$language]) === true )
			{
				return $serverLanguages[$language];
			}
		}

		return reset($serverLanguages);
	}