PHP works on some of my domains but not all

On one of my domains I get this error:

You don’t have permission to access /helloworld.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

On my other domains, it works. The permissions seem to be the same on the files and folders in both domains as do the settings in the Dreamhost panel.

Thanks for any help.

are any .htaccess files involved?

Yes, there were. Thanks for pointing me in the right direction.