Description:
------------
ZipArchive::OVERWRITE flag has no effect. Even if the flag is passed to ZipArchive::open method new files seem to be appended rather than overwrite the existing files.

Reproduce code:
---------------
<?php

$File = 'test.zip';

$zip = new ZipArchive;
$res = $zip->open($File, ZipArchive::CREATE);

if($res === true)
   {
      $zip->addFromString('foo.txt', 'foo bar foobar');
      $zip->close();

      echo "ok\n";
   }

else
   {
      echo "failed errno: ". $res ."\n\n";
   }

unset($zip);


$zip = new ZipArchive;
$res = $zip->open($File, ZipArchive::OVERWRITE);

if($res === true)
   {
      $zip->addFromString('bar.txt', 'foo bar foobar');
      $zip->close();

      echo "ok\n";
   }

else
   {
      echo "failed errno: ". $res ."\n\n";
   }

unset($zip);

Expected result:
----------------
Zip file containing bar.txt

Actual result:
--------------
Zip file containing foo.txt and bar.txt