Extracting data from a binary string

[Hagar]

I've the following problem:

I have a binary data string optained by uncompressing a file:
$uncompresseddata = gzuncompress($compresseddata);

Now I want to extract variables from this data (500K+), so normally I would use:
$values = unpack("Vvalue/C4byte", $uncompresseddata);

But I have 2 problems:
1. The data is spread over the whole string, at variable positions and the string is rather large, so I don't want to unpack it completely, I want to "random access" the data.
2. Some values are zero-terminted strings, so I can't use unpack to extract them. How do I obtain these ?

Thanks in advance.

[Hagar]

Maybe a little example would make it a bit clearer:

For the moment I use the following method:

function GetZString($pos)
{
  $str = "";
  do {
    $c = ord($Uncompresseddata[$pos]);
    $str .= chr($c);
    $pos++;
  } while ($c);
  return $str;
}

function Decode()
{
  ...
  for ($i = 0; $i < $blockcount; $i++)
  {
    $values = unpack("C" . $pos . "dummy/Cid/Vblocklength", $Uncompresseddata);
    $pos += 5;
    $names[$values['id']] = GetZString($pos);
    $pos += $values['blocklength'];
  }
  ...
}

This demonstrates my 2 problems.
- I use unpack to "move" to the correct position in $Uncompresseddata to extract other variables (like id and blocklenght). I suppose this is rather slow and uses a lot of memory when $pos becomes big.
- To extract a zero-terminated string I have to copy it byte by byte, which is also relatively slow ?

Are there any other solutions for these problems ?

Thanks.