There is *definately* a problem in PHP 4.0.6 with classes, inheritance, and included/required files.  Here is an example:

// ======================
// test.php:

<?php

 include("children.php");

?>

// ======================

// ======================
// children.php

<?php

 include_once("parent.php");

 class Child2 extends Child1
 {

 }

 class Child1 extends Parent
 {

 }

?>

// ======================

// ======================
// parent.php

<?php

 class Parent
 {

 }

?>

// ======================

You can try this out for yourself and see what I mean.  If you bring up "test.php" in your browser, you should receive this error:

Fatal error: Class child2: Cannot inherit from undefined class child1

Why does this happen, even though all files are being included correctly?  Well, I know one way of preventing the error, and that is by re-ordering the classes in "children.php" so that Child1() is listed first.  This removes the error, **however**, if the Parent() class is NOT in a separate file, and is actually part of the same file (i.e. "children.php"), the order of the class definitions does NOT matter.  Why is this?