This is bug #12032 but I don't have a password to add more 
info to it (didn't enter one, was rushed)

Yes,it seems to have to do with the calling of parent
object fuctions in files incuded from files that have been
included. Following are 3 php files that reproduce it,you
run the D.PHP file. If the a.php file is included in d.php
instead of b.php, there is no problem.
 
// -- A.PHP --
<?
class a
{
  function a()
  {
    ->_constructor();
  }
 
  function _constructor()
  {
  }
}
?>
// -- EOF --
 
// -- B.PHP --
<?
include("a.php");
 
class b extends a
{
  function _constructor()
  {
    parent::_constructor();
  }
}
 
class c extends b
{
  function _constructor()
  {
    parent::_constructor();
  }
}
?>
// -- EOF --
 
// -- D.PHP --
<?
{
  function _constructor()
  {
    parent::_constructor();
  }
}
 
class c extends b
{
  function _constructor()
  {
    parent::_constructor();
  }
}
?>
// -- EOF --
 
// -- D.PHP --
<?
  include("b.php");
   = new c;
?>
// -- EOF --