decodeAsciiHex

function decodeAsciiHex($input) {
    $output = "";

    $isOdd = true;
    $isComment = false;

    for($i = 0, $codeHigh = -1; $i < strlen($input) && $input[$i] != '>'; $i++) {
        $c = $input[$i];

        if($isComment) {
            if ($c == '\r' || $c == '\n')
                $isComment = false;
            continue;
        }

        switch($c) {
            case '\0': case '\t': case '\r': case '\f': case '\n': case ' ': break;
            case '%':
                $isComment = true;
            break;

            default:
                $code = hexdec($c);
                if($code === 0 && $c != '0')
                    return "";

                if($isOdd)
                    $codeHigh = $code;
                else
                    $output .= chr($codeHigh * 16 + $code);

                $isOdd = !$isOdd;
            break;
        }
    }

    if($input[$i] != '>')
        return "";

    if($isOdd)
        $output .= chr($codeHigh * 16);

    return $output;
}