Description:
------------
When setting variable variable values on a instance of a class with an overloading __set function changing another instance variable goes wrong.

(I'm reporting it for version 5.0.3 because the current cvs snapshot would not compile.)

Reproduce code:
---------------
http://dev.peytz.dk/~mk/setter.php
<?php
class Setter
{
    private $_fields = array(); // the data fields
    private $_changedFields = array(); // list of changed fields

    function __set($name, $value)
    {
        // set value
        $this->_fields[$name] = $value;
        // add to list of changed fields
        $this->_changedFields[] = $name;
    }
}
$foo = new Setter;
$foo->a = 1;
$var = "b";
$foo->$var = 2;
var_export($foo);
?>

Expected result:
----------------
class Setter {
  private $_fields = 
  array (
    'a' => 1,
    'b' => 2,
  );
  private $_changedFields = 
  array (
    0 => 'a',
    1 => 'b',
  );
}

Actual result:
--------------
class Setter {
  private $_fields = 
  array (
    'a' => 1,
    'b' => 2,
  );
  private $_changedFields = 
  array (
    0 => 'a',
    1 => '???',
  );
}