I am trying to create a PHP website with more than one administrator. I have some doubts

1. How can I check if an admin is logged in? I know I must use sessions for security reasons, but how can I recognize which one of the administrators is logged in?
2. Should I set a `$_SESSION['username']` variable when `username` performs login and send a cookie containing `username` too? This way, I know who is him (or pretends to be) and can check if `username` is actually logged in, checking `$_SESSION['username']` before showing pages. Is it secure?