What you have written is fine in terms of syntax. Its probably a case of not being able to find the file. As a general rule, if the file isnt in the same directory use:
I am used to the nice easy include statements of ASP, why can I not just include files like this?
Code:
<?php require_once('/db/DB.php'); ?>
You can include files like that, however, the file you are calling might not exist in that location as said above, what is the exact error you are reciveing?