What should be stored in a cookie for a login system?

Question

What is the best thing to store in a cookie to keep a persistent logged-in state?

I have seen many websites (and beginner tutorials!) that simply store something lik

Answer 1

The only thing that should be in a cookie is the sessionid generated by PHP (and this is done automatically when you use sessions). You should not store anything in the cookie except maybe a remembered username for the purpose of a "remember me" checkbox.