Description:
------------
multiple calls of fwrite() do not store data in the file, except last call, when opened large files (about 16mb) with fopen(filename, 'a+b')
This error does not appear when files opened with fopen(filename, 'r+b');

Reproduce code:
---------------
define(BUFFER_SIZE, 65536);
  function fCut($f, $pos, $size) {
    $fst = fstat($f);
    $fs = $fst["size"];
    if (($pos < 0) || ($pos >= $fs) || ($size <= 0)) return false;
    $ps = $pos;
    $sz = $fs - $ps - $size;
    while ($sz > 0) {
      $s = $sz; if ($s > BUFFER_SIZE) { $s = BUFFER_SIZE; }
      fseek($f, $ps + $size);
      $buf = fread($f, $s);
      fseek($f, $ps);
      $l = fwrite($f, $buf);
      $sz -= $s; $ps += $s;
    }
    ftruncate($f, max($fs - $size, $pos));
    return true;
  } // fCut - cutting $size bites in $f, from $pos

  $f = fopen(filename, 'a+b');
  flock($f, 2);
  fcut($f, 0, 123456);
  flock($f, 3);
  fclose($f);

Expected result:
----------------
123456 bites of file 'filename' must be deleted from position 0;

Actual result:
--------------
all data still the same, except block that transfered with last calls of fread/fwrite.
Note: this bug appears only in 'a+b' mode, but in 'r+b' all works fine.