evaluation of if condition

Hello, i've got a general question about the if condition: The problem is as follows. I'm using a set of classes with my scripts. But instead of including all the classes at the top of the script like [... main script ... include("class_a.php"); include("class_b.php"); instance_b = new class_b(); ... main script ... ] I am using the following construction of 'dynamic' class-loading: [... main script ... if (!$__CLASS_B__) { include("class_b.php"); } instance_b = new class_b(); ... main script ] and in class_b.php there is the following construction: [... class_b.php ... $__CLASS_B__ = true; if (!$__CLASS_A__) { include("class_a.php"); } class class_b extends class_a { ... } ... class_b.php ...] The big advandage is, that I only include the scripts that I really need and them I include just once. Scripts, that I don't use I don't include! The main problem is: I have to use a variable (like "$__CLASS_B__") in another script. What, if the variable changes (for some reason). Then I have to change all other scripts, too! A possible solution would be like: [... main script ... $include("class_b.php"); $instance_b = new class_b(); ... main script ...] [... class_b.php ... if (!$__CLASS_B__) { $__CLASS_B__ = true; include("class_a.php"); class class_b extends class_a { ... } } ... class_b.php ...] That would be very cool. I am sure, this would work, but I am uncertain, if this wouldn't make the script very big - and slow! For your opinions about that I would be very thankful. Bye, Moritz.