Parse accept headers per RFC-2616

parse_accept_header.php

<?php

# ref: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

function parse_accept_header($header, $field = "") {
    # save original value
    $orig_header = $header;

    $header = preg_replace('/[\r\n\t]/', '', $header);
    $field = strtolower(trim($field));

    # Remove, e.g. "Accept-Language: "
    $regexp = '/^([A-Za-z-]*):\s*/';

    if (preg_match($regexp, $header, $matches)) {
        $header = preg_replace($regexp, '', $header);

        if (empty($field)) {
            if (!empty($matches[1])) {
                $field = strtolower($matches[1]);
            } else {
                $field = 'accept';
            }

            $field = preg_replace('/^accept-?/', '', $field);
        }

        if (empty($header) && $field === 'encoding') {
            $header = 'identity';
        }
    } else {
        # Derive from $_SERVER if possible.
        if (false !== ($key = array_search($orig_header, $_SERVER, true))) {
            $field = strtolower($key);
            $field = str_replace('http_accept_', '', $field);

            if (empty($field)) {
                $field = 'accept';
            }
        }
    }

    $values = preg_split("/\s*,\s*/", $header);
    $parsed = array();
    $index = 0;

    foreach ($values as $value) {
        $value = trim($value, '"\'');
        $split = preg_split("/\s*;\s*/", $value);
        $type = array_shift($split);
        $format = "";
        $quality = 1.0;
        $major = $type;
        $minor = "";
        $params =  array();
        $wildcard = false;
        $sep = 'language' === $field ? '-' : '/';

        if (false !== strpos($type, $sep)) {
            list ($major, $minor) = explode($sep, $type, 2);
        }

        if ($major === '*') {
            $wildcard = true;
        }

        # Pull out `format` e.g. +xml
        if (false !== strpos($minor, '+')) {
            $format = ltrim(strstr($minor, '+'), '+');
        }

        while (sizeof($split) > 0) {
            $param = array_shift($split);

            if (false === strpos($param, '=')) {
                continue;
            }

            $paramName = strstr($param, '=', true);
            $paramValue = ltrim(strstr($param, '='), '=');

            if ($paramName === 'q') {
                $quality = (float) $paramValue;
            } else {
                $params[$paramName] = trim($paramValue, '"\'');
            }
        }

        $parsed[] = compact(
            'field', 'value', 'type', 'format', 'quality',
            'major', 'minor', 'wildcard', 'index', 'params'
        );

        $index += 1;
    }

    if (empty($parsed)) {
        return $parsed;
    }

    usort($parsed, function ($a, $b) {
        if ($a['quality'] === $b['quality']) {
            return $a['index'] > $b['index'] ? 1 : -1;
        } else {
            return $a['quality'] < $b['quality'] ? 1 : -1;
        }
    });

    return $parsed;
}

# ################### #
# ## Test ########### #
# ################### #

$headers = [
    0 => "text/plain; q=0.5, text/html, text/x-dvi; q=0.8, text/x-c",
    1 => "text/html, application/xml;q=0.9, application/xhtml+xml, image/png, image/webp, image/jpeg, image/gif, image/x-xbitmap, */*;q=0.1",
    2 => "Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5",
    3 => "text/css,*/*;q=0.1",
    4 => "da, en-gb;q=0.8, en;q=0.7",
    5 => "ISO-8859-1,utf-8;q=0.7,*;q=0.7",
    6 => "UTF-8,*;q=0.5",
    7 => "Accept: audio/*; q=0.2, audio/basic",
    8 => "gzip, deflate\t,sdch",
    9 => 'Accept-Encoding: *',
    10 => 'Accept-Encoding:',
    11 => 'Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0',
    12 => 'Accept-Charset:',
    13 => 'fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4',
    14 => '*;q=0.3,ISO-8859-1,utf-8;q=0.7',
    15 => '*;q=0.3,ISO-8859-1;q=0.7,utf-8;q=0.7',
    16 => '*;q=0.3,utf-8;q=0.7,ISO-8859-1;q=0.7',
    17 => 'accept: application/vnd.github+json',
    18 => 'Accept-Language: da, en-gb;q=0.8, en;q=0.7',
    19 => 'Accept-Ranges: bytes',
    20 => 'If-None-Match: "abcdefg12345678","apples"',
];

foreach($headers as $h) {
    var_dump(parse_accept_header($h));
    print '<HR>';
}