Lets say you have multiple developers working on a web project (php/mysql/linux). The database stores some sensitive information like account balances and credit cards. What methods do you normally use to keep the data as secure as possible.

I am not worried about keeping the data safe from outside attacks. I am worried about inside attacks. I need php scripts to display and manipulate this data.

Should I keep secure info in another database? I don't think you can query two databases at once, so that would make things more inefficient.