(The problem described is specific to 4.0.3 and 4.0.3pl1.
The scripts below work fine with 4.0.2.)

Let $a be an array kept in the session state.  Whenever
a reference $x to an element of $a falls of the end of a
script, the referenced array element is destroyed.
Putting an ``undef($x);'' at the end of the script solves
the problem.  The following two scripts demonstate the
problem:

BEGIN FILE test1.php
<?php
  session_register('a');
  $a = array();
  $a['foo'] = 'FOO';
?>
<html>
<a href="test2.php?<?=SID?>">link</a>
</html>
END FILE

BEGIN FILE test2.php
<?php
  session_register('a');
  $x = &$a['foo'];
?>
<html>
x=<?=$x?><br>
<a href="test2.php?<?=SID?>">link</a>
</html>
END FILE

Open test1.php in a browser and follow the link to
test2.php.  You'll see that $x is 'FOO'.  Now follow
that link again and $x is undefined.

The scripts work if you put an ``undef($x);'' at the
end of test2.php.