SOS adding php script into website adding to dreamweaver

The best place for help with editing a phpBB3 forum is in the phpBB support forum, not here.

Dreamweaver supports development with PHP, but you need to set up a PHP testing environment as described in this article in the Adobe Developer Connection. Even when you have set up a testing environment, you will have considerable difficulty customizing phpBB3 if you have no experience of working with PHP code. You cannot alter dynamically generated output in Dreamweaver Design view or Live View.

Unless you're willing to learn PHP and MySQL, you would be better advised getting a PHP programmer to do the job for you.