Here's the shortest possible example script I came up with,
which does exactly same as your multiple scripts plus
shows what really happens:

<?php

class data {var $f;}
session_start();
$phase = isset($_GET['phase']) ? $_GET['phase'] : 1;

switch ($phase) {
case 1:
  $_SESSION['foo'][0] = new data;
  $_SESSION['foo'][0]->f = 1;
  $_SESSION['foo'][1] = new data;
  $_SESSION['foo'][1]->f = 2;
  header("Location: test_web.php?phase=2");
  exit(1);
  break;

case 2:
  $_SESSION['bar'] = new data;
  $_SESSION['bar']->f = 3;
  header("Location: test_web.php?phase=3");
  exit(1);
  break;

case 3:
  $_SESSION['foo'][0] = $_SESSION['bar'];  // Here be bug
  header("Location: test_web.php?phase=4");
  exit(1);
  break;

case 4:
  $_SESSION['bar'] = 'foo';
  header("Location: test_web.php?phase=5");
  exit(1);
  break;

case 5:
  echo '<pre>';
  var_dump($_SESSION);
  echo '</pre>';
  break;
}

?>

Output is this:

array(2) {
  ["foo"]=>
  array(2) {
    [0]=>
    &string(3) "foo"
    [1]=>
    object(data)(1) {
      ["f"]=>
      int(2)
    }
  }
  ["bar"]=>
  &string(3) "foo"
}

Result: ['bar'] became a reference to ['foo'][0]
(Does NOT happen when using arrays instead of object)

Seems also to be fixed in HEAD (PHP 5.1.0-dev)

If it can do   With PHP4.X   I would like 
you to carry out fix.

bug #24485 seems to be same issue as this.

In my long code   Result at the time of performing 
by PHP4.3.9   The normal value outputted 
and I thought that it was the problem of PHP4.3.10.
It seems that it is the same when the code 
of bug #24485 is seen.
Are they things even if it becomes easier 
to find a bug in PHP4.3.10?

Assigning to Derick (another reference thing..)

Please try using this CVS snapshot:

  http://snaps.php.net/php4-STABLE-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php4-win32-STABLE-latest.zip

It is fixed.
Thank you.

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.