Well...,Dreamweaver, even if it supports syntax-highlighting of PHP source, isn't quite known as a PHP-development environment ; so I'm not sure it supports debugging of PHP code.
Instead, I would recommend you use another more PHP-Oriented IDE.
Below three all support debugging with the Xdebug extension:
1.Eclipse PDT (free -- not sure if it's maintained as much as it was a couple of month ago)
3.PHPStorm (not free)
Hope this helps.