Are you testing this locally or remotely? If you are testing remotely then you must ensure that the username and password you are entering are correct for the remote db. If you can test the db connection in DW then it seems that you should be ok.
As a test you can create a new php page and add a recordset to the page. After adding in this recordset go into the bindings tab and add some of the values from the recordset to the page then preview it. This will help to determine if it is just the pages that are having the problem or if there is a problem with the entire site. Let us know if you continue to have this issue and I can open a ticket for you to look into it further.
