Description:
------------
The last 4 bytes in GZIP file must contain the size of uncompressed data.
When you write in append mode, php writes just the size of the last chunk.

Test script:
---------------
<?
$str = '0123456789'; // 10 bytes

$f = gzopen('tmp.gz','w');
gzwrite($f, $str);
gzclose($f);

$f = gzopen('tmp.gz','a');
gzwrite($f, $str);
gzclose($f);
?>

# gzip -l tmp.gz

Expected result:
----------------
uncompressed
20

Actual result:
--------------
uncompressed
10