请教gzip压缩和解压

if (!function_exists('gzdecode')) {
function gzdecode ($data) {
    $flags = ord(substr($data, 3, 1));
    $headerlen = 10;
    $extralen = 0;
    $filenamelen = 0;
    if ($flags & 4) {
        $extralen = unpack('v' ,substr($data, 10, 2));
        $extralen = $extralen[1];
        $headerlen += 2 + $extralen;
    }
    if ($flags & 8) // Filename
        $headerlen = strpos($data, chr(0), $headerlen) + 1;
    if ($flags & 16) // Comment
        $headerlen = strpos($data, chr(0), $headerlen) + 1;
    if ($flags & 2) // CRC at end of file
        $headerlen += 2;
    $unpacked = @gzinflate(substr($data, $headerlen));
    if ($unpacked === FALSE)
        $unpacked = $data;
    return $unpacked;
}
}