Storing encryption key in $_SESSION vs $_COOKIE?

Question

Reading the tutorial here: https://github.com/defuse/php-encryption/blob/master/docs/Tutorial.md (scenario #2)

It is advised to store the encryption key in a session variable. I was wondering, is it safer to store the key in a cookie variable instead?

My assumption:

• $_SESSION is stored in the server
• $_COOKIE is stored in the client's device (browser)

Threat model:

• Against employees of hosting provider snooping data
• Against hackers (they will likely attack the server instead of a user's device)

Update:

Notes for clarification in comments for @SteffenUllrich. Below are the exact quotes in the tutorial.

SteffenUllrich: "Where the encrypted data are located..."

Alice wants to host Dave's web application, including its database, on her server

SteffenUllrich: "recommendation regarding storing the key in the session.."

save the Key object somewhere safe (like temporary memory-backed session storage)

Answer 1

The important thing is that key and encrypted data should not be stored at the same place, i.e. not both on disk, not both at the client side (no sending of encrypted data and storing the key in the cookie) etc. If the encrypted data remain on the server side both a cookie and a server side in-memory session object should be safe enough. Cookies might be even safer than a server side in-memory session object because they are more temporary, i.e. the client will provide the key if needed and if not needed the key will not exist on the server at all, not even in memory.