Maybe you dont need a database solution if all you want is to open up certain htdoc directories to customers or approved users. Put all the resource files on which you want to control access into a directory named 'resources' for instance. You can put html and php files in here as well. Then protect the directory with Basic or Digest authentication. Apache explanation is here: Authentication, Authorization, and Access Control.
When someone tries to browse to a file in 'resources' they get the little popup widow requiring userid/pswd. People would have to email you a request to get access or you could just map your customer file to .htaccess using their name as userid and invoice# as the password.