I can also reproduce this in Linux with PHP 5.1.6

Unfortunately, this is expected behavior.

Here is the logic path:

test.php => require_once classB.php
   classB.php required yet? no
classB.php => require_once classA.php
   classA.php required yet? no
classA.php => declare class A
classB.php => require_once classD.php
   classD.php required yet? no
classD.php => require_once classC.php
   classC.php required yet? no
classC.php => require_once classB.php
   classB.php required yet? YES, SKIP
classC extends ClassB => fatal error, classB doesn't exist.

There is a way around this, and it is to re-design your classes so 
they don't have such a complex relationship, or to re-order your 
require_once statements.  PHP can't be changed, as the parse order 
always requires parsing included files first in order to grab 
dependencies (parent classes), and so these are processed before the 
class declaration is even parsed.  Otherwise the simplest example of 
having two classes in separate files would fail.

For instance, if you were to place
require_once 'ClassD.php' after the classB declaration:

<?php
require_once 'ClassA.php';
class ClassB extends ClassA {}
require_once 'ClassD.php';
?>

your code sample will work.  Incidentally, PEAR makes use of this 
exact technique in order to handle complex inter-relationships.  
PEAR/Config.php has require_once statements at the end of the file 
after PEAR_Config class is created.

not a PHP bug => bogus.

Hi,

Actually something doesn't agree in the logic path in the case "extends ClassA" is removed in ClassB.php. The code is working in this case, though following the logic you described, it should fail.

Hi,

Actually something doesn't agree in the logic path in the case "extends ClassA" is removed in ClassB.php. The code is working in this case, though following the logic you described, it should fail.