万能的php，缺少的函数function gzdecode

奇怪的是却有 gzencode , 函数不对应，在php中极为少见。呵呵。

摘自php手册。

function gzdecode($data) {
  $len = strlen($data);
  if ($len < 18 || strcmp(substr($data,0,2),"\x1f\x8b")) {
   return null;  // Not GZIP format (See RFC 1952)
  }
  $method = ord(substr($data,2,1));  // Compression method
  $flags  = ord(substr($data,3,1));  // Flags
  if ($flags & 31 != $flags) {
   // Reserved bits are set -- NOT ALLOWED by RFC 1952
   return null;
  }
  // NOTE: $mtime may be negative (PHP integer limitations)
  $mtime = unpack("V", substr($data,4,4));
  $mtime = $mtime[1];
  $xfl  = substr($data,8,1);
  $os    = substr($data,8,1);
  $headerlen = 10;
  $extralen  = 0;
  $extra    = "";
  if ($flags & 4) {
   // 2-byte length prefixed EXTRA data in header
   if ($len - $headerlen - 2 < 8) {
     return false;    // Invalid format
   }
   $extralen = unpack("v",substr($data,8,2));
   $extralen = $extralen[1];
   if ($len - $headerlen - 2 - $extralen < 8) {
     return false;    // Invalid format
   }
   $extra = substr($data,10,$extralen);
   $headerlen += 2 + $extralen;
  }

  $filenamelen = 0;
  $filename = "";
  if ($flags & 8) {
   // C-style string file NAME data in header
   if ($len - $headerlen - 1 < 8) {
     return false;    // Invalid format
   }
   $filenamelen = strpos(substr($data,8+$extralen),chr(0));
   if ($filenamelen === false || $len - $headerlen - $filenamelen - 1 < 8) {
     return false;    // Invalid format
   }
   $filename = substr($data,$headerlen,$filenamelen);
   $headerlen += $filenamelen + 1;
  }

  $commentlen = 0;
  $comment = "";
  if ($flags & 16) {
   // C-style string COMMENT data in header
   if ($len - $headerlen - 1 < 8) {
     return false;    // Invalid format
   }
   $commentlen = strpos(substr($data,8+$extralen+$filenamelen),chr(0));
   if ($commentlen === false || $len - $headerlen - $commentlen - 1 < 8) {
     return false;    // Invalid header format
   }
   $comment = substr($data,$headerlen,$commentlen);
   $headerlen += $commentlen + 1;
  }

  $headercrc = "";
  if ($flags & 1) {
   // 2-bytes (lowest order) of CRC32 on header present
   if ($len - $headerlen - 2 < 8) {
     return false;    // Invalid format
   }
   $calccrc = crc32(substr($data,0,$headerlen)) & 0xffff;
   $headercrc = unpack("v", substr($data,$headerlen,2));
   $headercrc = $headercrc[1];
   if ($headercrc != $calccrc) {
     return false;    // Bad header CRC
   }
   $headerlen += 2;
  }

  // GZIP FOOTER - These be negative due to PHP's limitations
  $datacrc = unpack("V",substr($data,-8,4));
  $datacrc = $datacrc[1];
  $isize = unpack("V",substr($data,-4));
  $isize = $isize[1];

  // Perform the decompression:
  $bodylen = $len-$headerlen-8;
  if ($bodylen < 1) {
   // This should never happen - IMPLEMENTATION BUG!
   return null;
  }
  $body = substr($data,$headerlen,$bodylen);
  $data = "";
  if ($bodylen > 0) {
   switch ($method) {
     case 8:
       // Currently the only supported compression method:
       $data = gzinflate($body);
       break;
     default:
       // Unknown compression method
       return false;
   }
  } else {
   // I'm not sure if zero-byte body content is allowed.
   // Allow it for now...  Do nothing...
  }

  // Verifiy decompressed size and CRC32:
  // NOTE: This may fail with large data sizes depending on how
  //      PHP's integer limitations affect strlen() since $isize
  //      may be negative for large sizes.
  if ($isize != strlen($data) || crc32($data) != $datacrc) {
   // Bad format!  Length or CRC doesn't match!
   return false;
  }
  return $data;
}