Naming cookies - best practices

appname_meaningfulname

Keep in mind that this cookie is sent with every request, so imho, just use the smallest name you can, and document your code nicely.

I use whatever style the coding standards for the project call for.

Generally I prefer camelCase for naming schemes, but whichever one pays the bills is the one I'll go with.

Maybe you won't like my answer:

Don't use your own cookies but store data in server sessions. So you only need one cookie (to reference the session id) and how you name that plays no role.