The session name defaults to PHPSESSID. This is used as the name of the session cookie that is sent to the user's web browser / client. (Example: PHPSESSID=kqjqper294faui343o98ts8k77).
To hide this, call session_name() with the $name parameter set to a generic name, before calling session_start(). Example:
session_name("id");
session_start();
Cheers.
