session_name() returns the name
of the current session. If
name is specified, the name of the current session
is changed to its value.
The session name references the session id in cookies
and URLs. It should contain only alphanumeric characters; it
should be short and descriptive (i.e. for users with enabled
cookie warnings). The session name is reset to the default
value stored in session.name at
request startup time. Thus, you need to call session_name() for every request (and before
session_start() or
session_register() are called).
See also the session.name
configuration directive.